[go] Remove workaround in cpp11_decltype.i

This case was fixed by 7a8c9fdfa8.

(SWIG/Go still has problems wrapping constant strings in some cases
though - the example in #2724 still fails.)

Examples/test-suite/cpp11_decltype.i

@@ -12,12 +12,6 @@
 #endif
 %}
 
-#ifdef SWIGGO
-// FIXME: SWIG/Go tries to wrap this by generating code which tries to
-// assign a const char* value to a char* variable.
-%ignore should_be_string;
-#endif
-
 %inline %{
   class A {
   public: