[Go] Documentation cleanup of obsolete 'callback' and 'extend' examples.

After commit 17b1c1c (pull request 447; issue 418) the 'callback' and 'extend'
examples have been removed in favor of the 'director' example.

Examples/go/callback/example.i

@@ -1,11 +0,0 @@
-/* File : example.i */
-%module(directors="1") example
-%{
-#include "example.h"
-%}
-
-/* turn on director wrapping Callback */
-%feature("director") Callback;
-
-%include "example.h"
-