treewide: Remove usage of remaining redundant platform compatability stuff

Want to get this out of here for 18.09, so it can be deprecated
thereafter.

pkgs/development/tools/misc/libtool/libtool2.nix

@@ -1,5 +1,4 @@
 { stdenv, fetchurl, m4, perl, help2man
-, buildPlatform, hostPlatform
 }:
 
 stdenv.mkDerivation rec {
@@ -26,7 +25,7 @@ stdenv.mkDerivation rec {
 
   # Don't run the native `strip' when cross-compiling.  This breaks at least
   # with `.a' files for MinGW.
-  dontStrip = hostPlatform != buildPlatform;
+  dontStrip = stdenv.hostPlatform != stdenv.buildPlatform;
 
   meta = {
     description = "GNU Libtool, a generic library support script";