There currently exists two pure virtual classes for monitoring: ReplicaSetMonitor and ReplicaSetMonitorInterface. The latter was introduced in SERVER-45962 to support switching between the new StreamableReplicaSetMonitor implementation and the legacy ScanningReplicaSetMonitor. The scanning monitor has since been removed in SERVER-62079, but the redundant interface was left behind. We should remove it.

Confusingly, both of the remaining monitoring protocols (kStreamable and kSdam) are both implemented by the sole remaining ReplicaSetMonitor implementation, StreamableReplicaSetMonitor. We could consider collapsing this hierarchy down into a single, concrete ReplicaSetMonitor implementation to help avoid this confusion in the future.