Reapply "Disable webkit2gtk integration"

This reverts commit 8823d47.

Due to the way the default values of withFoo flags are specified and
the way Emacs variants are overridden, default values of those flags
in this overlay may be incorrect.

This commit is a workaround.  We should do a proper fix later.

overlays/emacs.nix

@@ -16,7 +16,7 @@ let
       super.emacs
       ([
 
-        (drv: drv.override ({ srcRepo = true; } // args))
+        (drv: drv.override ({ srcRepo = true; withXwidgets = false; } // args))
 
         (
           drv: drv.overrideAttrs (