Remove workaround for IsUsableSpell() that (mis)used IsSpellOverlayed().
Remove workaround for IsUsableSpell() that (mis)used IsSpellOverlayed().

We can now map from a spell ID to the index within the spellbook, so the
Blizzard API function IsUsableSpell() called with index, bookType will
always return the right information, regardless of if a spell was replaced
due to a proc.

ultijlam/ovale,eXhausted/Ovale,ultijlam/ovale,eXhausted/Ovale,ultijlam/ovale,eXhausted/Ovale,Xeltor/ovale