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
- getting rid of magic numbers and some cleanup
tankGUID should be initialized too like https://github.com/TrinityCore/TrinityCore/pull/13008/files#diff-4b573d3aa466a86dd7fd2413dc86ed3cR56