Clear active spells from actor when he disappears from scene

According to my testing, Morrowind cleans up active effects from actor in these cases:

1. When his cell is unloaded
2. When he is disabled via script
3. When he is teleported to an inactive cell

Note that effects are cleaned up for player's followers, which are moved to new cell with player.

I suppose that Morrowind does it to do not update magic effects for actors in inactive cells.

This feature may be a quite hard ro implement in OpenMW since our current approach to move actor to another cell is to "remove old actor instance from scene and spawn a new one".