stdenv cross adapter: Get rid if `selfNativeBuildInput`

It is finally no longer used anywhere

Fixes #30587

pkgs/stdenv/adapters.nix

@@ -75,22 +75,11 @@ rec {
   in stdenv // {
     mkDerivation =
       { nativeBuildInputs ? []
-      , selfNativeBuildInput ? args.crossAttrs.selfNativeBuildInput or false
       , ...
       } @ args:
 
-      let
-        # *BuildInputs exists temporarily as another name for
-        # *HostInputs.
-
-        # The base stdenv already knows that nativeBuildInputs and
-        # buildInputs should be built with the usual gcc-wrapper
-        # And the same for propagatedBuildInputs.
-        nativeDrv = stdenv.mkDerivation args;
-      in
         stdenv.mkDerivation (args // {
           nativeBuildInputs = nativeBuildInputs
-            ++ stdenv.lib.optional selfNativeBuildInput nativeDrv
               # without proper `file` command, libtool sometimes fails
               # to recognize 64-bit DLLs
             ++ stdenv.lib.optional (hostPlatform.config == "x86_64-w64-mingw32") pkgs.file