We're keeping the core idea that, before we remove the frames-to-be-destroyed
from the continuation chain, their textruns need to be disconnected/destroyed.
However, nsContinuingTextFrame::DestroyFrom tries to optimize when the
destroying frames that aren't mentioned in the userdata for the textrun, and
certain other conditions are met; we need a similar optimization here. It's
simpler here because the other conditions are definitely met, since all the
text for the frames being deleted has already been consumed and reflowed by
previous frames.
We don't need the TEXT_STYLE_MATCHES_PREV_CONTINUATION state bit anymore
because nsContinuingTextFrame::DestroyFrom will never see any textruns when
called via RemoveEmptyInFlows.
