PlacemarkLayout: avoid having deleted VisiblePlacemarks in m_paintOrder

setCacheData() runs qDeleteAll on m_visiblePlacemarks but did not remove
them from m_paintOrder. This could cause random crashes (semi-reproducible
at startup by moving/clicking the mouse before the map is loaded).
(cherry picked from commit 78980c378a7eff47fd68ad4b013f0d5c346315ef)