Scripts/Ulduar: Exclude Ignis' main target from the Slag Pot target selector

Closes #10516
Scripts/ScarletMonastery: Summon the Headless Horseman once the Call the Headless Horseman is quest complete

By @cr4st
Closes #13273
DB/Creature: Fix 2 startup errors
Core/AHBot: Filter out from AHBot items that can't be looted
Merge branch '3.3.5' of https://github.com/TrinityCore/TrinityCore