lib/gnat: Remove Compiler_Unit_Warning pragmas

'pragma Compiler_Unit_Warning' is removed upstream:
https://gcc.gnu.org/git/?p=gcc.git&a=search&h=HEAD&st=commit&s=pragma+Compiler_Unit_Warning

Fix:
    GCC        libgnat-x86_32/lib/gnat/interfac.o
interfac.ads:36:08: warning: unrecognized pragma "Compiler_Unit_Warning" [-gnatwg]

Change-Id: I6d7efab132441dd3cc62a53b7322e9fd355e5059
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73162
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

src/lib/gnat/s-stoele.adb

@@ -29,8 +29,6 @@
 --                                                                          --
 ------------------------------------------------------------------------------
 
-pragma Compiler_Unit_Warning;
-
 with Ada.Unchecked_Conversion;
 
 package body System.Storage_Elements is