Fix ps display for IO workers.

This code must have missed a memo about the backend type description
being supplied automatically these days, and was duplicating that
information.

Before: "io worker io worker: N"
After:  "io worker N"

src/backend/storage/aio/method_worker.c

@@ -385,7 +385,7 @@ IoWorkerMain(const void *startup_data, size_t startup_data_len)
 	/* also registers a shutdown callback to unregister */
 	pgaio_worker_register();
 
-	sprintf(cmd, "io worker: %d", MyIoWorkerId);
+	sprintf(cmd, "%d", MyIoWorkerId);
 	set_ps_display(cmd);
 
 	/* see PostgresMain() */