doc: remove double mention of custom workspace

In the GOPATH section of the 'How To Write Go Code' document, it is
mentioned two times in the span of a few lines that one can set GOPATH
to a custom workspace path. The two paragraphs say basically the same
thing, and they both link to golang.org/wiki/SettingGOPATH, so I'm
quite sure the duplication is not intentional.

This change deletes the second occurrence.

Change-Id: I16f8bb8657041a23ed272eacf9adbc5637e8e34a
Reviewed-on: https://go-review.googlesource.com/c/go/+/197839Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/code.html

@@ -166,11 +166,6 @@ To learn more about the <code>GOPATH</code> environment variable, see
 <a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>'go help gopath'</code></a>.
 </p>
 
-<p>
-To use a custom workspace location,
-<a href="https://golang.org/wiki/SettingGOPATH">set the <code>GOPATH</code> environment variable</a>.
-</p>
-
 <h3 id="ImportPaths">Import paths</h3>
 
 <p>