remove model.id and model.cid in _indexRemove()

Untested, but surely we want this to mirror `_indexAdd`, no?  Seems like a memory leak and cause for inaccurate membership tests...

Also, what about changes to the model's id?  I don't have time to look into this at present, but IIRC Backbone has some special handling of this scenario.  I'll take a closer look later this evening and make sure this case is covered...