xenPackages.xen_4_8-vanilla: stop overriding cc

Nothing requires gcc49 in this version.

pkgs/applications/virtualization/xen/packages.nix

···
   };
 
   xen_4_8-vanilla = callPackage ./4.8.nix {
-    # At the very least included seabios and etherboot need gcc49,
-    # so we have to build all of it with gcc49.
-    stdenv = overrideCC stdenv gcc49;
-
     meta = {
       description = "vanilla";
       longDescription = ''