doc: remove broken link for keygen

Remove broken link to deprecated keygen element.

Replacing the link with something to the relevant
part of the HTML spec might be OK, but I don't think
it's necessary. People who need to know will find it
and everyone else should be discouraged from using
this.

Support for the keygen element was removed from Chrome in 2017
and removed from Firefox in 2019. We might consider deprecating
and removing support ourselves, or at least marking the API
as legacy.

Refs: https://caniuse.com/?search=keygen
PR-URL: #48176
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>

doc/api/crypto.md

@@ -78,7 +78,7 @@ added: v0.11.8
 -->
 
 SPKAC is a Certificate Signing Request mechanism originally implemented by
-Netscape and was specified formally as part of [HTML5's `keygen` element][].
+Netscape and was specified formally as part of HTML5's `keygen` element.
 
 `<keygen>` is deprecated since [HTML 5.2][] and new projects
 should not use this element anymore.
@@ -6016,7 +6016,6 @@ See the [list of SSL OP Flags][] for details.
 [Caveats]: #support-for-weak-or-compromised-algorithms
 [Crypto constants]: #crypto-constants
 [HTML 5.2]: https://www.w3.org/TR/html52/changes.html#features-removed
-[HTML5's `keygen` element]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/keygen
 [JWK]: https://tools.ietf.org/html/rfc7517
 [NIST SP 800-131A]: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar1.pdf
 [NIST SP 800-132]: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf