Add support for RCv3 to remove_from_load_balancer 

This is, next to #772, the other big one. That one should be reviewed first.
Er sorry, I realize that seems silly, since nothing else is supposed to return `None`, but if there were a bug in `_remove_from_clb` or something where it didn't return the right thing, this test would still pass.

I'm not sure if it's necessary to make sure the above assertions in `_fake_remove_from_rcv3` are actually made, or if that's just overkill?