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/libraries/libpng/12.nix

@@ -1,8 +1,6 @@
-{ stdenv, fetchurl, zlib
-, buildPlatform, hostPlatform
-}:
+{ stdenv, fetchurl, zlib }:
 
-assert hostPlatform == buildPlatform -> zlib != null;
+assert stdenv.hostPlatform == stdenv.buildPlatform -> zlib != null;
 
 stdenv.mkDerivation rec {
   name = "libpng-1.2.57";