fix occasional crash on dragging and dropping pages in slidesorters

pages go into the cache, and sometimes they get deleted before the
cache gets processed. Remove deleted pages when they go away

Change-Id: I291072a8541f4ca36979e9914975d81cc23a9497
(cherry picked from commit abe9d1463282690313aaf91d2a54011d10b900b9)
(cherry picked from commit 026e9335d792c6557255f064960e0ef6d28728e0)
Reviewed-on: https://gerrit.libreoffice.org/7053
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>