Creature/Scripting: Move CreatureAI::CanRespawn to CreatureScript::CanSpawn. Now also applies to initial spawn. Dynamic spawning prep.

(cherry picked from commit b3d44d6c366c7e4f12a6ce30ee5cf6b9bff68a06)

Game/Scripting: Follow-up to b3d44d6. Fix script selection for creatures on difficulty > 0.
(cherry picked from commit 91c2b3162d350ab1a09053574ad67f45f089175b)

Build fix.
(cherry picked from commit a9f1151f2ca56394e315d0b07741c240ef59c2ff)

Fine, this time I actually built it. Promise.
(cherry picked from commit 0c2c88ed5025d9b51b6cbdf0603ceaf1ea54c80a)

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -496,14 +496,6 @@ void BossAI::_EnterCombat()
     ScheduleTasks();
 }
 
-bool BossAI::CanRespawn()
-{
-    if (instance && instance->GetBossState(_bossId) == DONE)
-        return false;
-
-    return true;
-}
-
 void BossAI::TeleportCheaters()
 {
     float x, y, z;