Bug 1729824 - Remove unused layout.animated-image-layers.enabled. r=hiro,layout-reviewers,emilio

Differential Revision: https://phabricator.services.mozilla.com/D125007

layout/reftests/invalidation/reftest.list

@@ -10,8 +10,8 @@
 == 1243409-1.html 1243409-1-ref.html
 skip == test-image-layers.html test-image-layers-ref.html # Bug 1067360
 skip == test-image-layers-multiple-displayitem.html test-image-layers-ref.html # Bug 1067360
-pref(layout.animated-image-layers.enabled,true) skip-if(gtkWidget) == test-animated-image-layers.html test-animated-image-layers-ref.html
-pref(layout.animated-image-layers.enabled,true) skip-if(gtkWidget) == test-animated-image-layers-background.html test-animated-image-layers-ref.html
+skip-if(gtkWidget) == test-animated-image-layers.html test-animated-image-layers-ref.html
+skip-if(gtkWidget) == test-animated-image-layers-background.html test-animated-image-layers-ref.html
 == box-shadow-border-radius.html box-shadow-border-radius-ref.html
 == filter-userspace-offset.svg?offsetContainer=rect filter-userspace-offset.svg
 == filter-userspace-offset.svg?offsetContainer=use filter-userspace-offset.svg

modules/libpref/init/StaticPrefList.yaml

@@ -6682,12 +6682,6 @@
   value: false
   mirror: always
 
-# Whether we should layerize all animated images (if otherwise possible).
-- name: layout.animated-image-layers.enabled
-  type: bool
-  value: false
-  mirror: always
-
 # One of several prefs affecting the maximum area to pre-render when animating
 # a large element on the compositor.
 # This pref enables transform (and transform like properties) animations on a