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/libxslt/default.nix

@@ -1,7 +1,6 @@
 { stdenv, fetchurl, libxml2, findXMLCatalogs, python2
-, buildPlatform, hostPlatform
 , cryptoSupport ? false
-, pythonSupport ? buildPlatform == hostPlatform
+, pythonSupport ? stdenv.buildPlatform == stdenv.hostPlatform
 }:
 
 assert pythonSupport -> python2 != null;
@@ -22,7 +21,7 @@ stdenv.mkDerivation rec {
   patches = stdenv.lib.optional stdenv.isSunOS ./patch-ah.patch;
 
   # fixes: can't build x86_64-unknown-cygwin shared library unless -no-undefined is specified
-  postPatch = optionalString hostPlatform.isCygwin ''
+  postPatch = optionalString stdenv.hostPlatform.isCygwin ''
     substituteInPlace tests/plugins/Makefile.in \
       --replace 'la_LDFLAGS =' 'la_LDFLAGS = $(WIN32_EXTRA_LDFLAGS)'
   '';