webkitgtk: Disable Hydra build

This package takes an excessive amount of time to build (e.g., right
now Hydra is showing eight concurrent builds of webkitgtk, some of
them running for more than 6 hours). This may also delay channel
updates.

pkgs/development/libraries/webkitgtk/default.nix

@@ -18,6 +18,7 @@ stdenv.mkDerivation rec {
     homepage = "http://webkitgtk.org/";
     license = licenses.bsd2;
     platforms = platforms.linux;
+    hydraPlatforms = [];
     maintainers = with maintainers; [ iyzsong koral ];
   };