Prevent crash when Model gets deleted while a KeepMailboxOpenTask is still alive

The switch to a QPointer seems rather big, but let's hope that the overhead of
this is more than compensated for by a reduced potential of memory corruption.

BUG: 336090
REVIEW: 118671
Remove duplicated calls

These actions are already performed within abort() which we explicitly call, so
there's no point in repeating them once again from here.
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (.desktop file)
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (.desktop file)