Remove outdated docs on serverless builds and runtime configs

[Siggnja]

Fixes #15906

This should solve #15906. It simply removes a block that states runtime configuration for serverless builds are not supported.

[lfades]

LGTM Thank you!

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
buildDuration	13.5s	13.5s	-72ms
nodeModulesSize	66 MB	66 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.629	2.641	⚠️ +0.01
/ avg req/sec	951.05	946.56	⚠️ -4.49
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.655	1.656	0
/error-in-render avg req/sec	1510.42	1509.74	⚠️ -0.68

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
677f882d2ed8..afbf.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-76d73eb..95d8.js gzip	6.73 kB	6.73 kB	✓
webpack-ccf5..276a.js gzip	751 B	751 B	✓
Overall change	56.8 kB	56.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
677f882d2ed8..dule.js gzip	6.11 kB	6.11 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-0b6ca7d..dule.js gzip	5.81 kB	5.81 kB	✓
webpack-10c7..dule.js gzip	751 B	751 B	✓
Overall change	51.8 kB	51.8 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
polyfills-75..1629.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_app-874bd8a..0103.js gzip	1.28 kB	1.28 kB	✓
_error-fa39c..ec40.js gzip	3.45 kB	3.45 kB	✓
hooks-585f07..95a3.js gzip	887 B	887 B	✓
index-c7b63f..fc02.js gzip	227 B	227 B	✓
link-e97733c..af21.js gzip	1.3 kB	1.3 kB	✓
routerDirect..ebc7.js gzip	284 B	284 B	✓
withRouter-2..db68.js gzip	284 B	284 B	✓
Overall change	7.72 kB	7.72 kB	✓

Client Pages Modern

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_app-97e743e..dule.js gzip	626 B	626 B	✓
_error-b4004..dule.js gzip	2.3 kB	2.3 kB	✓
hooks-696209..dule.js gzip	387 B	387 B	✓
index-a4dd74..dule.js gzip	226 B	226 B	✓
link-fbb1d2e..dule.js gzip	1.25 kB	1.25 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-1..dule.js gzip	282 B	282 B	✓
Overall change	5.36 kB	5.36 kB	✓

Client Build Manifests

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_buildManifest.js gzip	272 B	272 B	✓
_buildManife..dule.js gzip	280 B	280 B	✓
Overall change	552 B	552 B	✓

Rendered Page Sizes

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
index.html gzip	945 B	945 B	✓
link.html gzip	953 B	953 B	✓
withRouter.html gzip	939 B	939 B	✓
Overall change	2.84 kB	2.84 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
buildDuration	15.2s	15.2s	⚠️ +6ms
nodeModulesSize	66 MB	66 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
677f882d2ed8..afbf.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-76d73eb..95d8.js gzip	6.73 kB	6.73 kB	✓
webpack-ccf5..276a.js gzip	751 B	751 B	✓
Overall change	56.8 kB	56.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
677f882d2ed8..dule.js gzip	6.11 kB	6.11 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-0b6ca7d..dule.js gzip	5.81 kB	5.81 kB	✓
webpack-10c7..dule.js gzip	751 B	751 B	✓
Overall change	51.8 kB	51.8 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
polyfills-75..1629.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_app-874bd8a..0103.js gzip	1.28 kB	1.28 kB	✓
_error-fa39c..ec40.js gzip	3.45 kB	3.45 kB	✓
hooks-585f07..95a3.js gzip	887 B	887 B	✓
index-c7b63f..fc02.js gzip	227 B	227 B	✓
link-e97733c..af21.js gzip	1.3 kB	1.3 kB	✓
routerDirect..ebc7.js gzip	284 B	284 B	✓
withRouter-2..db68.js gzip	284 B	284 B	✓
Overall change	7.72 kB	7.72 kB	✓

Client Pages Modern

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_app-97e743e..dule.js gzip	626 B	626 B	✓
_error-b4004..dule.js gzip	2.3 kB	2.3 kB	✓
hooks-696209..dule.js gzip	387 B	387 B	✓
index-a4dd74..dule.js gzip	226 B	226 B	✓
link-fbb1d2e..dule.js gzip	1.25 kB	1.25 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-1..dule.js gzip	282 B	282 B	✓
Overall change	5.36 kB	5.36 kB	✓

Client Build Manifests

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_buildManifest.js gzip	272 B	272 B	✓
_buildManife..dule.js gzip	280 B	280 B	✓
Overall change	552 B	552 B	✓

Serverless bundles

	vercel/next.js canary	Siggnja/next.js docs-runtime-config-serverless	Change
_error.js	1.02 MB	1.02 MB	✓
404.html	4.18 kB	4.18 kB	✓
hooks.html	3.82 kB	3.82 kB	✓
index.js	1.02 MB	1.02 MB	✓
link.js	1.06 MB	1.06 MB	✓
routerDirect.js	1.05 MB	1.05 MB	✓
withRouter.js	1.05 MB	1.05 MB	✓
Overall change	5.2 MB	5.2 MB	✓

Commit: bb92ae5