Remove requiresForcedStyleRecalcPropagation()

The world has changed a lot since this code was added way back in
https://trac.webkit.org/changeset/59876 3 years ago. Shadow DOM handles
style changes fine. We no longer need this virtual hook for Forced style
propagation into form controls.

I manually verified that the test that was added in that patch still
passes even though we've since disabled it in
https://trac.webkit.org/changeset/89688 2 years ago, only a year after
the patch landed that needed this test.

Review URL: https://codereview.chromium.org/65303004

git-svn-id: svn://svn.chromium.org/blink/trunk@161758 bbb929c8-8fbe-4397-9dbb-9b2b20218538