Remove threaded parser settings.

All tests work with the threaded parser now, so there's no need to
expose any settings to disable it for test purposes.

BUG=308321,366354

Review URL: https://codereview.chromium.org/295913005

git-svn-id: svn://svn.chromium.org/blink/trunk@174746 bbb929c8-8fbe-4397-9dbb-9b2b20218538