[Ada] Remove an unnecessary local constant

[Pierre-Marie de Rodat]

Code cleanup related to preanalysis in GNATprove mode; behaviour is
unaffected.

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

gcc/ada/

        * sem_ch6.adb (Analyze_Expression_Function): A local Expr
        constant was shadowing a global constant with the same name and
        the same value.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -532,7 +532,6 @@ package body Sem_Ch6 is
 
          declare
             Decls : List_Id            := List_Containing (N);
-            Expr  : constant Node_Id   := Expression (Ret);
             Par   : constant Node_Id   := Parent (Decls);
             Typ   : constant Entity_Id := Etype (Def_Id);