[Go] add typemaps for std::string*

Fixes #2214

Examples/test-suite/go_director_inout.i

@@ -120,15 +120,6 @@ type GoRetStruct struct {
     $1.assign(tmp->p, tmp->p + tmp->n);
 }
 
-%typemap(directorin) std::string * (_gostring_ temp) {
-    $input = &temp;
-    $input->p = (char *) $1->data();
-    $input->n = $1->size();
-}
-%typemap(directorargout) std::string * {
-    $1->assign($input->p, $input->p + $input->n);
-}
-
 %feature("director") MyClass;
 
 %inline