style: Remove pref for CSS Containment (layout.css.contain.enabled) r…

…=AlaskanEmily

(Since we've been shipping with it default-enabled for a while now.)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1466008#c9 through #c13 for
notes on the reftest.list change.

Differential Revision: https://phabricator.services.mozilla.com/D71861

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

@@ -612,9 +612,7 @@ ${helpers.predefined_type(
     engines="gecko",
     animation_value_type="none",
     flags="CREATES_STACKING_CONTEXT FIXPOS_CB",
-    gecko_pref="layout.css.contain.enabled",
     spec="https://drafts.csswg.org/css-contain/#contain-property",
-    enabled_in="chrome",
 )}
 
 // Non-standard