[z2013] Let PossessedSummons despawn on Channeled-Aura interrupt

Maybe this mechanic should also be used for normal possessed mobs, but then their special handling must be removed
Also not entirely sure if this a great place for this code to be - but I didn't catch a better suiting one

(based on commit 65d4f03)

Signed-off-by: Nighoo <nighoo@overninethousand.de>

(cleaned version of 17453f7)

(based on commit [s1619] - 2a21e00)