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

This is required to support RCv3.
I don't have any strong opinions either way, except that if bulk requests is the only reason to do this in `delete_server`, then the optimization may not be worth the extra effort, since we're doing this one per group anyway, and a group can have at most 5 load balancers configured. 

Whereas with convergence we may be converging multiple tenant groups at once (or all really, since we're getting all the data for that tenant at a time anyway).