style: Drop layout.css.image-orientation.enabled pref.

It was enabled by default in bug 825771.

Differential Revision: https://phabricator.services.mozilla.com/D15704

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

@@ -77,6 +77,5 @@ ${helpers.single_keyword(
     products="gecko",
     gecko_enum_prefix="StyleImageOrientation",
     animation_value_type="discrete",
-    gecko_pref="layout.css.image-orientation.enabled",
     spec="https://drafts.csswg.org/css-images/#propdef-image-orientation",
 )}