Remove a bit of the aStyleDisplay gunk that shouldn't be needed because
of two reasons:

 * Stylo is faster / has the style display one pointer-chase away, as
   opposed to the old style system.

 * We now check the MAY_BE_TRANSFORMED bit first now, and we deal with
   SVG-transformed frames, so for non-transformed frames IsTransformed
   should just be a bit-check now.

Differential Revision: https://phabricator.services.mozilla.com/D109511