Scripts/Onyxia Lair: small rework

- fix crash on boss death
- avoid abusing during phase switch
- added better facing through trigger
- corrected some timers
- corrected inhabit type
use _JustDied() and rename Summons -> summons, because you can delete Summons(me)

check https://github.com/TrinityCore/TrinityCore/blob/master/src/server/game/AI/ScriptedAI/ScriptedCreature.cpp#L458

SummonedCreatureDespawn() is also not needed, everything is in BossAI