Fix Map destructor incorrectly deleting MapItemViews

Currently the views are explicitly destroyed, leading to
issues, in particular for views declared inside the Map.
Change it to simply remove these views from the map.
Child views will be destroyed in ~QObject.

[ChangeLog] Fixed crash when destroying Maps containing
MapItemViews.

Change-Id: Iff9b1afd6b17b55671b1f999b1bf69f172a05483
Fixes: QTBUG-69195
Fixes: QTBUG-74337
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>