firefox: drop patch that's applied already

It's a bit weird that noone's noticed,
but I guess it's because of a merge or rebase.

pkgs/applications/networking/browsers/firefox/packages.nix

@@ -28,12 +28,6 @@ rec {
 
     patches = nixpkgsPatches ++ [
       ./no-buildconfig.patch
-      # fix build with rust >= 1.29 and firefox < 63
-      # https://bugzilla.mozilla.org/show_bug.cgi?id=1479540
-      (fetchpatch {
-        url = "https://github.com/mozilla/gecko-dev/commit/eec0d4f8714e6671402d41632232ef57348e65c4.patch";
-        sha256 = "1cjaqx811bcnp8b6z16q25csaclaic3b11q45ck02srd99n8qp0j";
-      })
     ];
 
     extraNativeBuildInputs = [ python3 ];