[Ada] Cleanup unnecessary shadowing in expansion of attribute Old

[Marc Poulhiès via Gcc-patches]

Code cleanup; semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * exp_attr.adb (Expand_N_Attribute_Reference [Attribute_Old]):
        Remove unnecessary local constant that was shadowing another
        constant with the same initial value.
diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -4884,7 +4884,6 @@ package body Exp_Attr is
       ---------
 
       when Attribute_Old => Old : declare
-         Typ     : constant Entity_Id := Etype (N);
          CW_Temp : Entity_Id;
          CW_Typ  : Entity_Id;
          Decl    : Node_Id;