Remove unused "acceleratedCompositingForScrollableFramesEnabled" setting

Remove unused "acceleratedCompositingForScrollableFramesEnabled" setting
https://bugs.webkit.org/show_bug.cgi?id=127402

Reviewed by Anders Carlsson.

Source/WebCore:

compositing/iframes/iframe-composited-scrolling.html is updated because
there are no usages in WebCore.

* page/Settings.in:

LayoutTests:

* compositing/iframes/iframe-composited-scrolling.html:
Removed acceleratedCompositingForScrollableFramesEnabled changes because
it is dead code in WebCore side.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@162507 268f45cc-cd09-0410-ab3c-d52691b4dbfc