Remove obsolete comment.

In earlier versions of the sync rep patch, waiters removed themselves from
the queue, but now walsender removes them before doing the wakeup.

Report by Fujii Masao.

src/include/replication/walsender.h

@@ -74,8 +74,7 @@ typedef struct
 
 	/*
 	 * Current location of the head of the queue. All waiters should have
-	 * a waitLSN that follows this value, or they are currently being woken
-	 * to remove themselves from the queue. Protected by SyncRepLock.
+	 * a waitLSN that follows this value. Protected by SyncRepLock.
 	 */
 	XLogRecPtr	lsn;