fdo#87199: sw: fix root cause of a11y crash when merging cells

Commit f9eff2a402a4cd28d7dbfb6ce27cbf96b31e576f is not quite right
because it will leave the mpNext chain unreachable; that could perhaps
be imporoved by calling RemoveFromLayout(), but...

Actually the problem is basically that one of the deleted SwCellFrms
points to a SwTableBox with getRowSpan() -1 (because it has been merged)
and thus IsInCoveredCell() returns true and that causes ~SwCellFrm() to
skip disposing the SwAccessible stuff, so the SwCellFrm is still
contained in the SwAccessibleMap.

Because it's rather hard to prevent this sort of thing in general,
better change SwAccessibleMap::Dispose() to assume that if it found its
way into the SwAccessibleMap it should be disposed and removed.

Change-Id: Ib4cec6924cb026ae30bdac6857957adf237b4d70
(cherry picked from commit 296e8b597c141b6b54cbf943871d6a6820c1779d)
Reviewed-on: https://gerrit.libreoffice.org/13532
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>