treewide: Remove gnat support.

See discussion in https://github.com/NixOS/nixpkgs/commit/6ac7b19c978e951c124c5ea434c94f95f593888e.

pkgs/build-support/cc-wrapper/add-flags.sh

@@ -9,7 +9,6 @@ var_templates_list=(
     NIX+CFLAGS_LINK
     NIX+CXXSTDLIB_COMPILE
     NIX+CXXSTDLIB_LINK
-    NIX+GNATFLAGS_COMPILE
 )
 var_templates_bool=(
     NIX+ENFORCE_NO_NATIVE
@@ -51,10 +50,6 @@ if [ -e @out@/nix-support/cc-cflags ]; then
     NIX_@infixSalt@_CFLAGS_COMPILE="$(< @out@/nix-support/cc-cflags) $NIX_@infixSalt@_CFLAGS_COMPILE"
 fi
 
-if [ -e @out@/nix-support/gnat-cflags ]; then
-    NIX_@infixSalt@_GNATFLAGS_COMPILE="$(< @out@/nix-support/gnat-cflags) $NIX_@infixSalt@_GNATFLAGS_COMPILE"
-fi
-
 if [ -e @out@/nix-support/cc-ldflags ]; then
     NIX_@infixSalt@_LDFLAGS+=" $(< @out@/nix-support/cc-ldflags)"
 fi