dar: don't use llvm 12

Builds with llvm 11 now, this no longer appears to be necessary.

pkgs/tools/backup/dar/default.nix

@@ -1,7 +1,6 @@
 args @ {
   lib,
   stdenv,
-  llvmPackages_12, # Anything newer than 11
   fetchzip,
   which,
   attr,
@@ -22,12 +21,6 @@ args @ {
   CoreFoundation,
 }:
 
-let
-  # Fails to build with clang-11 on Darwin:
-  # error: exception specification of overriding function is more lax than base version
-  stdenv = if args.stdenv.isDarwin then llvmPackages_12.stdenv else args.stdenv;
-in
-
 stdenv.mkDerivation rec {
   version = "2.7.13";
   pname = "dar";