[rennes][servers] remove no more used servers

data/grid5000/sites/rennes/servers/ceph-mon0.json

-{
-  "kind": "virtual",
-  "network_adapters": {
-    "default": {
-      "ip": "172.16.111.158"
-    }
-  },
-  "type": "server",
-  "uid": "ceph-mon0"
-}
\ No newline at end of file