doc/go1.10: remove ReverseProxy TODO

No longer needs to be done.

Updates #23009
Updates #21255

Change-Id: I78e9e29a923dc03dea89ff3a5bf60f2e0bd0c0aa
Reviewed-on: https://go-review.googlesource.com/86476Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.10.html

@@ -1180,13 +1180,6 @@ if passed an invalid (non-3-digit) status code.
 </p>
 </dl>
 
-<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
-<dd>
-<p>
-TODO: ReverseProxy and back end errors and ModifyResponse.
-</p>
-</dl>
-
 <dl id="net/mail"><dt><a href="/pkg/net/mail/">net/mail</a></dt>
 <dd>
 <p>