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/fontconfig/default.nix

@@ -1,6 +1,5 @@
 { stdenv, substituteAll, fetchurl
 , pkgconfig, freetype, expat, libxslt, gperf, dejavu_fonts
-, hostPlatform
 }:
 
 /** Font configuration scheme
@@ -40,13 +39,13 @@ stdenv.mkDerivation rec {
   buildInputs = [ expat ];
 
   configureFlags = [
-    "--with-arch=${hostPlatform.parsed.cpu.name}"
+    "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}"
     "--with-cache-dir=/var/cache/fontconfig" # otherwise the fallback is in $out/
     "--disable-docs"
     # just <1MB; this is what you get when loading config fails for some reason
     "--with-default-fonts=${dejavu_fonts.minimal}"
   ] ++ stdenv.lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
-    "--with-arch=${hostPlatform.parsed.cpu.name}"
+    "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}"
   ];
 
   enableParallelBuilding = true;