Fix: cluster: Prevent search of unames from attempting to create node entries for unknown nodes

When removing an offline node with "crm_node --remove",  the corresponding "node_state" will be deleted from cib, which will trigger the following in  te_legacy_update_diff():
abort_transition(INFINITY, tg_restart, "Transient attribute: removal", aborted);

Though  it'll add the removed node back into the node cache due to:
const char *uname = crm_peer_uname(ID(reason));