rpc : early register backend devices (#11262)

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

ref: #10609

ggml/src/ggml-backend-impl.h

@@ -208,7 +208,6 @@ extern "C" {
 
     // Internal backend registry API
     GGML_API void ggml_backend_register(ggml_backend_reg_t reg);
-    GGML_API void ggml_backend_device_register(ggml_backend_dev_t device);
 
     // Add backend dynamic loading support to the backend