[Ada] Remove unused initialization with New_List

[Pierre-Marie de Rodat]

Code cleanup; behaviour is unaffected. The extra initialization most
likely come from a initial version of the patch, before the limit of
parameters to a subsequent call to New_List was discovered.

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

gcc/ada/

        * exp_ch3.adb (Build_Slice_Assignment): Remove unused
        initialization.
diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -4388,7 +4388,7 @@ package body Exp_Ch3 is
 
       declare
          Spec    : Node_Id;
-         Formals : List_Id := New_List;
+         Formals : List_Id;
 
       begin
          Formals := New_List (