Fix default meta tag being added in AMP mode

[ijjk]

This was causing the "conflicting head tag" message to be logged. Also updated tests to check for this.

Closes: #7240

[github-actions]

Stats from current PR

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	zeit/next.js canary	ijjk/next.js fix/conflicting-meta	Change
Build Duration	15.6s	14.7s	-928ms
node_modules Size	39.1 MB	39.1 MB	⚠️ +277 B
Total Bundle (main, webpack, commons) Size	209 kB	209 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.5 kB	68.5 kB	✓
Client _app Size	2.6 kB	2.6 kB	✓
Client _app gzip Size	1.07 kB	1.07 kB	✓
Client _error Size	8.19 kB	8.19 kB	✓
Client _error gzip Size	3.12 kB	3.12 kB	✓
Client pages/index Size	288 B	288 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	4.78 kB	4.78 kB	✓
Client pages/link gzip Size	2.09 kB	2.09 kB	✓
Client pages/routerDirect Size	411 B	411 B	✓
Client pages/routerDirect gzip Size	296 B	296 B	✓
Client pages/withRouter Size	393 B	393 B	✓
Client pages/withRouter gzip Size	280 B	280 B	✓
Client main Size	23.3 kB	23.3 kB	✓
Client main gzip Size	7.63 kB	7.63 kB	✓
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.5 kB	59.5 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	769 B	769 B	✓
Base Rendered Size	1.34 kB	1.34 kB	✓
Build Dir Size	802 kB	802 kB	⚠️ +72 B

Click to expand serverless stats

	zeit/next.js canary	ijjk/next.js fix/conflicting-meta	Change
Build Duration	16.3s	16.4s	⚠️ +22ms
node_modules Size	39.1 MB	39.1 MB	⚠️ +277 B
Total Bundle (main, webpack, commons) Size	209 kB	209 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.5 kB	68.5 kB	✓
Client _app Size	2.6 kB	2.6 kB	✓
Client _app gzip Size	1.07 kB	1.07 kB	✓
Client _error Size	8.19 kB	8.19 kB	✓
Client _error gzip Size	3.12 kB	3.12 kB	✓
Client pages/index Size	288 B	288 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	4.78 kB	4.78 kB	✓
Client pages/link gzip Size	2.09 kB	2.09 kB	✓
Client pages/routerDirect Size	411 B	411 B	✓
Client pages/routerDirect gzip Size	296 B	296 B	✓
Client pages/withRouter Size	393 B	393 B	✓
Client pages/withRouter gzip Size	280 B	280 B	✓
Client main Size	23.3 kB	23.3 kB	✓
Client main gzip Size	7.63 kB	7.63 kB	✓
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.5 kB	59.5 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	769 B	769 B	✓
Serverless pages/link Size	343 kB	343 kB	⚠️ +111 B
Serverless pages/link gzip Size	88.9 kB	88.9 kB	⚠️ +19 B
Serverless pages/index Size	334 kB	334 kB	⚠️ +111 B
Serverless pages/index gzip Size	86.5 kB	86.5 kB	⚠️ +20 B
Serverless pages/_error Size	335 kB	335 kB	⚠️ +111 B
Serverless pages/_error gzip Size	86.2 kB	86.3 kB	⚠️ +20 B
Serverless pages/routerDirect Size	334 kB	334 kB	⚠️ +111 B
Serverless pages/routerDirect gzip Size	86.5 kB	86.5 kB	⚠️ +18 B
Serverless pages/withRouter Size	334 kB	334 kB	⚠️ +111 B
Serverless pages/withRouter gzip Size	86.6 kB	86.6 kB	⚠️ +21 B
Build Dir Size	2.37 MB	2.37 MB	⚠️ +592 B