Core/Dungeon Finder: Fix group (5) unable to do new dungeon after finished last dungeon

- Some optimizations here and there
- Drop unused columns related to dungeon rewards
- Simplify Group reward. All people inside the dungeon should get the reward, no matter how far it's from the boss

sql/updates/2013_01_09_world_lfg_dungeon_rewards.sql

@@ -0,0 +1,6 @@
+ALTER TABLE `lfg_dungeon_rewards`
+  DROP `firstMoneyVar`,
+  DROP `firstXPVar`,
+  DROP `otherMoneyVar`,
+  DROP `otherXPVar`;
+