Remove ad-hoc "untyped" guard from wrong_return_type

The string comparison `actual_ty == "untyped"` was no longer needed.
This guard was originally needed when `check_match` was used
(1a933d6), but became dead code when `check_match` was replaced
with `typecheck` in 3251cbc.

Author: mame

lib/typeprof/core/graph/box.rb

@@ -311,7 +311,6 @@ def run0(genv, changes)
 
     def wrong_return_type(f_ret_show, changes)
       actual_ty = @a_ret.show
-      return if actual_ty == "untyped" # XXX: too ad-hoc?
       msg = "expected: #{ f_ret_show }; actual: #{ actual_ty }"
       case @node
       when AST::ReturnNode