Fix Knock Off getting boosted for unremovable items

http://www.smogon.com/forums/threads/battle-mechanics-research.3489239/page-51#post-5155710
The current implementation of Knock Off gets boosted even when the item is unremovable (other than megastones), whereas it should only get the boost if the item can be removed (ignoring abilities etc).
Things that shouldn't give Knock Off a boost:
 - Drives on Genesect
 - Plates on Arceus formes
 - Griseous Orb
 - Mega Stones
This removes the erroneous boost for the above items.