ei: drop ei_seat_bind_capability() and the unbind equivalent

This function is almost always wrong, very few clients will want to bind to a single capability. Having this function means clients will use it to bind caps one-by-one, causing the EIS implementation to create (an later destroy) devices with capabilities that are about to be bound again anyway. Better to have an API that encourages clients to bind all at once.

EIS implementation could avoid this by using a pingpong rountrip in response to a bind call, but removing this API is likely going to have the same utility.