... and ensure that they are removed from the mpShapeMap.

Commit 76c549eb added all sub-shapes of
group shapes to SwAccessibleMap::mpShapeMap.
It's not immediately obvious why this is necessary, given that
AccessibleShape will probably create these on demand.

It turns out that the AccessibleShapes are not only disposed from
SwAccessible code but also from svx code,
in particular the call to ViewForwarderChanged() in
SwAccessibleContext::ChildrenScrolled() leads to visibility checks.

Thus the RemoveGroupContext() may fail to remove the sub-shapes from the
mpShapeMap and the assertion in ~SwAccessibleMap is triggered.

Change-Id: I6d9c3d816f5521960855d5e6a563bec49d8030b8