Clean up no-longer-used enableAccessibility methods on WebAXObject

Depends on https://codereview.chromium.org/497273002

BUG=406622

Review URL: https://codereview.chromium.org/501613002

git-svn-id: svn://svn.chromium.org/blink/trunk@181153 bbb929c8-8fbe-4397-9dbb-9b2b20218538