Core/Auras: don't save auras casted by items

- Drop useless field from DB (itemGuid was being saved, but never used, see Prepared Statements)
- This type of auras (like Water Bucket, Precious Ribbon, etc) will be recasted on login anyways.

This reverts commit 16cfd59f5b

sql/updates/characters/3.3.5/2016_10_30_00_characters.sql

@@ -0,0 +1,2 @@
+DELETE FROM `character_aura` WHERE `itemGuid` != 0;
+ALTER TABLE `character_aura` DROP COLUMN `itemGuid`;