[rennes][servers] remove no more used servers

data/grid5000/sites/rennes/servers/cargo0.json

-{
-  "alias": [
-
-  ],
-  "kind": "physical",
-  "network_adapters": {
-    "bmc": {
-      "ip": "172.17.111.40"
-    },
-    "default": {
-      "ip": "172.16.111.40"
-    }
-  },
-  "type": "server",
-  "uid": "cargo0"
-}
\ No newline at end of file