chromium: remove preferLocalBuild

It's another attempt to fix chromium builds.

See http://hydra.nixos.org/build/26086977/nixlog/4/raw

Unpacking sources is actually taking more than 2h so build fails.
Instead, rather build it remotely and then copy over the output as
we don't have limits for download time.

See 089bdce621 for reference

cc @aszlig

pkgs/applications/networking/browsers/chromium/source/default.nix

@@ -74,8 +74,6 @@ in stdenv.mkDerivation {
     cat $opensslPatches | patch -p1 -d "$bundled/openssl/openssl"
   '';
 
-  preferLocalBuild = true;
-
   passthru = {
     inherit version channel;
     plugins = fetchurl binary;