style: Remove layout.css.webkit-appearance.enabled.

I don't think we want to keep the ugly widget hacks forever. Let me know if
you'd rather keep the property behind a pref but I don't think there's a point
in doing that.

Differential Revision: https://phabricator.services.mozilla.com/D62649

components/style/properties/longhands/box.mako.rs

@@ -633,7 +633,7 @@ ${helpers.predefined_type(
     "Appearance",
     "computed::Appearance::None",
     engines="gecko",
-    alias="-webkit-appearance:layout.css.webkit-appearance.enabled",
+    alias="-webkit-appearance",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-appearance)",
     animation_value_type="discrete",
     gecko_ffi_name="mAppearance",