Combat/Threat: Split ThreatManager::NotifyDisengaged off from ThreatManager::ClearAllThreat. NotifyDisengaged signifies intent to clear the engagement flag, and should only be called from AI. Fixes #23490.

# Conflicts:
#	src/server/game/AI/CreatureAI.cpp
#	src/server/game/AI/ScriptedAI/ScriptedFollowerAI.cpp
#	src/server/scripts/EasternKingdoms/ScarletEnclave/chapter5.cpp
#	src/server/scripts/Kalimdor/zone_desolace.cpp
#	src/server/scripts/Northrend/zone_borean_tundra.cpp

src/server/game/Entities/Unit/Unit.cpp

@@ -9194,7 +9194,6 @@ void Unit::setDeathState(DeathState s)
     if (s != ALIVE && s != JUST_RESPAWNED)
     {
         CombatStop();
-        GetThreatManager().ClearAllThreat();
         ClearComboPointHolders();                           // any combo points pointed to unit lost at it death
 
         if (IsNonMeleeSpellCast(false))