DelegateModel leaks m_cache entries when removing incubating elements

It seems that when the delegate model updates as a result of the input model, entries in m_cache aren't deleted, resulting in the delegate model slowing more and more as the model keeps changing.

To reproduce:
1- patch QtLocation with https://codereview.qt-project.org/243943
2- open the sample 71264_BrokenMIV.tar.gz from QTBUG-71264
3- zoom out a bit to let the map populate
4- pan to verify the initial speed of the view (at every movement of the map, the model changes)
5- use the Ctrl+P shortcut many times, this will trigger the leak
6- pan again

at this point the speed of the application should have decreased considerably. Debugging it reveals that in the delegate model used inside QDeclarativeGeoMapItemView, the
list m_cache contains a big number of entries with index set to -1

A proposed fix, https://codereview.qt-project.org/#/c/244309/, solves the leak but introduces a crash when using transitions in the view (unsure about the reason), suggesting that it is probably the incorrect approach to take