fix(DB/Core): "Battle for the Undercity" buffs (#1986)

- Cast the buffs from Thrall and Wrynn on themselves instead of the player. This prevents interrupting spellcasting of the player during battle.
- Override "EnterEvadeMode" for Thrall and Wrynn AI in order to prevent removing their auras on evade.
- Remove the spell script "spell_undercity_buffs" as it is not needed anymore (the players only get the aura when near Thrall or Wrynn)

data/sql/updates/pending_db_world/rev_1560980987673005006.sql

@@ -0,0 +1,3 @@
+INSERT INTO `version_db_world` (`sql_rev`) VALUES ('1560980987673005006');
+
+DELETE FROM `spell_script_names` WHERE `ScriptName` = 'spell_undercity_buffs';