firefox-bin: fix license

The Mozilla Trademark Policy is not a license, and applies equally to
our own source builds of Firefox, so it doesn't make sense to mark the
package as unfree because of that.

Quoting <about:license>:

> Binaries of this product have been made available to you by the
> Mozilla Project under the Mozilla Public License 2.0 (MPL).

Since all this does is download a large binary and wrap it, there's
still no point in it ending up in cache.nixos.org, so disable it on
Hydra now that Hydra would otherwise try to build it.

(cherry picked from commit 6a71c7eb1a)

pkgs/applications/networking/browsers/firefox-bin/default.nix

@@ -198,11 +198,9 @@ stdenv.mkDerivation {
   meta = with lib; {
     description = "Mozilla Firefox, free web browser (binary package)";
     homepage = "http://www.mozilla.org/firefox/";
-    license = {
-      free = false;
-      url = "http://www.mozilla.org/en-US/foundation/trademarks/policy/";
-    };
+    license = licenses.mpl20;
     platforms = builtins.attrNames mozillaPlatforms;
+    hydraPlatforms = [];
     maintainers = with maintainers; [ taku0 lovesegfault ];
   };
 }