doc: remove expiration from PGP key in security.html

The key had expired earlier this year. Simply resigned it with no
expiration, so it maintains the same fingerprint.

Removed the encouragement to use PGP above the fold. We trust the
security of our mail system, so it's really only there for people that
want it.

Also removed the individual keys, as they were never used, and both Adam
and I have access to the security@golang.org key anyway.

Change-Id: Icc5ad6dfb4f0b52128a59a080b7f270b20d3c520
Reviewed-on: https://go-review.googlesource.com/c/go/+/211177Reviewed-by: Katie Hockman <katie@golang.org>

doc/security.html

@@ -20,7 +20,6 @@ This mail is delivered to a small security team.
 Your email will be acknowledged within 24 hours, and you'll receive a more
 detailed response to your email within 72 hours indicating the next steps in
 handling your report.
-For critical problems, you can encrypt your report using our PGP key (listed below).
 </p>
 
 <p>
@@ -42,8 +41,8 @@ security team directly:
 </p>
 
 <ul>
-<li>Primary security coordinator: <a href="mailto:filippo@golang.org">Filippo Valsorda</a> (<a href="https://keybase.io/filippo/pgp_keys.asc">public key</a>).</li>
-<li>Secondary coordinator: <a href="mailto:agl@golang.org">Adam Langley</a> (<a href="https://www.imperialviolet.org/key.asc">public key</a>).</li>
+<li>Primary security coordinator: <a href="mailto:filippo@golang.org">Filippo Valsorda</a>.</li>
+<li>Secondary coordinator: <a href="mailto:agl@golang.org">Adam Langley</a>.</li>
 <li>If you receive no response, mail <a href="mailto:golang-dev@googlegroups.com">golang-dev@googlegroups.com</a> or use the <a href="https://groups.google.com/forum/#!forum/golang-dev">golang-dev web interface</a>.</li>
 </ul>
 
@@ -130,7 +129,6 @@ use PGP for critical security reports.
 
 <pre>
 -----BEGIN PGP PUBLIC KEY BLOCK-----
-Comment: GPGTools - https://gpgtools.org
 
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 -----END PGP PUBLIC KEY BLOCK-----
 </pre>