[release-branch.go1.8] doc: remove mentions of yacc tool

It was removed in CL 27325.

Fixes #20431

Change-Id: I6842851444186e19029d040f61fdf4f87a3103a6
Reviewed-on: https://go-review.googlesource.com/43771Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit deebd8fe273df2de2d590ee41ae1155c521219e9)
Reviewed-on: https://go-review.googlesource.com/43772Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/cmd.html

@@ -22,8 +22,6 @@ using the go <code>tool</code> subcommand, such as <code>go tool vet</code>.
 This style of invocation allows, for instance, checking a single source file
 rather than an entire package: <code>go tool vet myprogram.go</code> as
 compared to <code>go vet mypackage</code>.
-Some of the commands, such as <code>yacc</code>, are accessible only through
-the go <code>tool</code> subcommand.
 </p>
 
 <p>
@@ -95,12 +93,6 @@ gofmt</a> command with more general options.</td>
 calls whose arguments do not align with the format string.</td>
 </tr>
 
-<tr>
-<td><a href="/cmd/yacc/">yacc</a></td>
-<td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
-<td>Yacc is a version of yacc that generates parsers implemented in Go.</td>
-</tr>
-
 </table>
 
 <p>