server: add SSE ping interval (#24013)

tools/server/server-queue.h

@@ -169,8 +169,6 @@ struct server_response_reader {
     bool cancelled = false;
     int polling_interval_seconds;
 
-    const int64_t time_start_ms = ggml_time_ms();
-
     // tracking generation state and partial tool calls
     // only used by streaming completions
     std::vector<task_result_state> states;