removing a replica should delete the PRS entry from the hostnode and not from overseer

For further improvements based on https://issues.apache.org/jira/browse/SOLR-16414, there are places that we should remove PRS entry operations from overseer:

1. In `ZkController#unregister`, the PRS hosting node should remove the PRS entries from ZK, as overseer no longer does so as in the change of https://github.com/apache/solr/pull/1019/files#diff-b08497f972c47127009b59dc49de322698312c08ee53684d6e916db256db16a2L149
2. We should as well remove the PRS entries ops from Overseer in `SliceMutator#addReplica` https://github.com/apache/solr/blob/c96c78132d77974adaec62ffaaf4b819f76f49a9/solr/core/src/java/org/apache/solr/cloud/overseer/SliceMutator.java#L112-L123