Remove unused AddReplicaCmd properties

AddReplicaCmd currently has logic to read and use a parameter called 'skipCreateReplicaInClusterState' (which defaults to false). Unfortunately, CollectionsHandler doesn't actually pass that parameter.

It looks like the parameter was added for use by the autoscaling "simulation" framework (see SOLR-11285). When that was cut from Solr later on the parameter was removed from CollectionsHandler, but the corresponding change to AddReplicaCmd was never made.

We should remove this line from AddReplicaCmd, and any other references to that now-defunct param.