Core: Units that are Summoned by other units linked to a game_event are not removed from the world when the event ends

[Killyana]

Description: When a creature/gob linked to a game_event summon an other unit, the summoned unit will not be removed from the world when the event ends.

Example: Flame Keeper from Midsummer Fire Festival use this spell https://woehead.way-of-elendil.fr/?spell=47123 to summon a Festival Scorchling https://woehead.way-of-elendil.fr/?npc=26520

The summoned npc is not a pet or a guardian, it will not despawn if the Flame Keeper is killed or despawn. Probably for this reason when the event Midsummer Fire Festival ends this Scorchling remains spawned until the server restarts.

Steps to reproduce the problem:

1. .go c 94710
2. .event start 1
3. .event stop 1
4. the summoned Festival Scorchling will not despawn

Branch(es): both

TC rev. hash/commit: rev. 948b590