Prevent combat drop when creature charms expire while the creature is in combat

[bladezLH]

• The combat end timer will begin to tick once a unit's hated list is empty. Therefore, we should ensure that the caster is still on the threat list after a successful charm
• We should potentially restore a unit's threat list from before the charm rather than wiping it and only adding the caster
• Removed Kel'Thuzad specific MC handling
• Ensure Kel'Thuzad properly enters combat with the instance

[ratkosrb]

You can use ToCreature(caster) here instead.