[4.3.4] Spells: Dispel effects can always be casted

Since 4.X dispel effects no longer check if there's something to dispel on the target.

Quote from GMs:
```
In Cataclysm we are raising the mana costs, making it possible to waste 
mana by casting a dispel when there is nothing to dispel.
```
Source: http://www.mmo-champion.com/threads/706936-Cataclysm-Dispel-Mechanics?p=7584466

I'm leaving it commented since we don't really know if this mechanic will eventually return.