Removing unused role IncrementorRole from blink.

IncrementorRole is not used anywhere in blink as a role for html element or ARIA role to chromium.

This CL depends on https://codereview.chromium.org/714063002/

BUG=385014

Review URL: https://codereview.chromium.org/703793003

git-svn-id: svn://svn.chromium.org/blink/trunk@185268 bbb929c8-8fbe-4397-9dbb-9b2b20218538