picoev: extract common code to a trace_fd/1 function, cleanup

vlib/picoev/loop_default.c.v

@@ -83,18 +83,13 @@ fn (mut pv Picoev) poll_once(max_wait_in_sec int) int {
 				if C.FD_ISSET(target.fd, &writefds) != 0 {
 					read_events |= picoev_write
 				}
-
 				if read_events != 0 {
-					$if trace_fd ? {
-						eprintln('do callback ${target.fd}')
-					}
-
+					trace_fd('do callback ${target.fd}')
 					// do callback!
 					unsafe { target.cb(target.fd, read_events, &pv) }
 				}
 			}
 		}
 	}
-
 	return 0
 }