doc: remove a Google+ link since it doesn't exist anymore

Change-Id: I692229a5597a6114a7f4261f7b59c58bf20f3e2b
GitHub-Last-Rev: ab173354caa63345d38a625705aea0c660cfdb9b
GitHub-Pull-Request: golang/go#32660
Reviewed-on: https://go-review.googlesource.com/c/go/+/182677Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/help.html

@@ -59,9 +59,6 @@ for important announcements, such as the availability of new Go releases.
 <h3 id="twitter"><a href="https://twitter.com/golang">@golang at Twitter</a></h3>
 <p>The Go project's official Twitter account.</p>
 
-<h3 id="pluscom"><a href="https://plus.google.com/communities/114112804251407510571">Go+ community</a></h3>
-<p>A Google+ community for Go enthusiasts.</p>
-
 <h3 id="reddit"><a href="https://reddit.com/r/golang">golang sub-Reddit</a></h3>
 <p>
 The <a href="https://reddit.com/r/golang">golang sub-Reddit</a> is a place