Map::DeleteActor/RemoveActor: also clear the area resref

After escaping Irenicus's dungeon Imoen would correctly be removed
from the map. If you left the area and returned, she would, however,
reappear.
Map::PurgeArea: clear the map reference for all actors

Other code may try to reference it, and we're about to drop the
map from memory.

Fixes a crash in the circus in BG2 after having defeated Kalah.
The cutscene script uses MoveGlobalsTo which tries to access the
map this way.