Core/Scripts: remove OnDummyEffect hook/sOnDummyEffect ai hook

- Duplicated logic never used, sometimes only ScriptMgr version was called, sometimes only AI
- They only encourage bad scripting practices
- You can still use OnSpellHit or a SpellScript

(cherry picked from commit b6b59f6c239bc0259c92c28bf43bbb50573330b5)

src/server/game/AI/SmartScripts/SmartAI.cpp

@@ -830,12 +830,6 @@ void SmartAI::sQuestReward(Player* player, Quest const* quest, uint32 opt)
     GetScript()->ProcessEventsFor(SMART_EVENT_REWARD_QUEST, player, quest->GetQuestId(), opt);
 }
 
-bool SmartAI::sOnDummyEffect(Unit* caster, uint32 spellId, SpellEffIndex effIndex)
-{
-    GetScript()->ProcessEventsFor(SMART_EVENT_DUMMY_EFFECT, caster, spellId, (uint32)effIndex);
-    return true;
-}
-
 void SmartAI::SetCombatMove(bool on)
 {
     if (mCanCombatMove == on)