rpc : early register backend devices (#11262)

Early register RPC devices and do not propagate RPC specifics in the
llama model structures.

ref: #10609

src/llama-model.h

@@ -323,8 +323,6 @@ struct llama_model {
     // gguf metadata
     std::unordered_map<std::string, std::string> gguf_kv;
 
-    std::vector<std::string> rpc_servers;
-
     // list of devices used in this model
     std::vector<ggml_backend_dev_t> devices;