DelegatingMarker.remove() should call map.onRemove(this) *before* real.remove()

=> DelegatingGoogleMap.onRemove(DelegatingMarker marker) calls
createdMarkers.remove(marker.getReal().getMarker()), but getMarker()
always returns null at this point if the marker is removed before
calling onRemove, since LazyMarker#remove nulls it.