Use load balancer configuration, not load balancer id, in job-based LB removal logic

This is required to support RCv3.
That's an excellent question :) I'm open to suggestions :)

I think I started doing this because of the "RCv3 allows bulk requests" argument. Obviously, since `remove_from_load_balancer` is singular, you can't have those neat bulk requests. The only thing that has access to all of them is `delete_server`, and here they're conveniently grouped by type, so it would be really easy to pass it on to a generic RCv3 bulk remove from load balancers function.

That said, I'm increasingly convinced my initial instinct of keeping the high-level `remove_from_load_balancer` and just updating it to support RCv3 is a good idea: we've already seen how the test suite may underspecify some behavioral constraints. In that light, it might be more appropriate to go for the "I can't believe it's not Cloud Butter!" approach, rather than the "It's not whiskey. It's not bourbon. It's RCv3." approach.

Thoughts?

Obviously, if we go for the "just do `remove_load_balancer`" approach, this thing would be thrown out.