rb_method_name_error: delete unused code

If you look at the code flow (break -> goto), this assignment never
makes any sense.  Should just remove.

I _guess_ this behaviour is unintended.  Original code at commit
4dc1a2180946ab793adee5eb235fc4ee8fa4cefe did something.  It might be
the code flow that is buggy.  However rubyspec already includes this
particular edge case at ruby/core/module/undef_method_spec.rb.  I don't
think we can change the way it is any longer.

proc.c

@@ -1817,7 +1817,6 @@ rb_method_name_error(VALUE klass, VALUE str)
 	  case T_MODULE:
 	  case T_CLASS:
 	    c = obj;
-	    s = MSG("");
             break;
           default:
 	    break;