praefect: Remove logic to handle maintenance-style replication events
This MR removes the logic to handle maintenance-style replication events. We haven't created those anymore since %15.0, and have stubbed out the logic handling them already. So we really shouldn't be seeing any of them anymore, but to be sure that they're gone we also add a migration to prune any stale ones.
Closes #4185 (closed).
Edited by Patrick Steinhardt