Captions: Delete *all* cc708 windows in CC708Reader::Reset().

Refs #12227.

Signed-off-by: Jim Stichnoth <jstichnoth@mythtv.org>
(cherry picked from commit e97f4f59806a32421a2d2ab4c79549286c56b6ab)

Conflicts:

	mythtv/libs/libmythtv/cc708reader.cpp