Empty implementation of grpc_use_signal since not actually needed anymore

src/core/lib/iomgr/ev_posix.cc

@@ -395,4 +395,6 @@ void grpc_pollset_set_del_fd(grpc_pollset_set* pollset_set, grpc_fd* fd) {
   g_event_engine->pollset_set_del_fd(pollset_set, fd);
 }
 
+void grpc_use_signal(int signum) {}
+
 #endif  // GRPC_POSIX_SOCKET_EV