Remove beacons when the badgers get shot down

Done as suggested in https://github.com/OpenRA/OpenRA/pull/6497#issuecomment-56092420 .

Would fix #6184 .

PS: Sorry for the new PR, but I can't reopen the old one.
This (and also the check inside `ParatroopersPower`) need to be inside the `if (!aircraftInRange.Any(kv => kv.Value))` to prevent bad behavior when there are multiple attack/drop aircraft.