rtpproxy: fix compiler warnings

> rtpproxy.c: In function 'rtpproxy_rpc_list':
> rtpproxy.c:627:6: warning: unused variable 'len' [-Wunused-variable]
>   int len;
>       ^~~
> rtpproxy.c:626:6: warning: unused variable 'id_len' [-Wunused-variable]
>   int id_len;
>       ^~~~~~
> rtpproxy.c:625:8: warning: variable 'string' set but not used [-Wunused-but-set-variable]
>   char *string, *id;
>         ^~~~~~

src/modules/rtpproxy/rtpproxy.c

@@ -622,13 +622,8 @@ static void  rtpproxy_rpc_list(rpc_t* rpc, void* ctx)
 {
 	struct rtpp_set *rtpp_list;
 	struct rtpp_node *crt_rtpp;
-	char *string, *id;
-	int id_len;
-	int len;
 	void *vh;
 
-	string = id = 0;
-
 	if(rtpp_set_list ==NULL)
 		return;