xfs: xfs_buf: drop useless LIST_HEAD

Drop LIST_HEAD where the variable it declares has never
been used.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
identifier x;
@@
- LIST_HEAD(x);
  ... when != x
// </smpl>

Fixes: 26f1fe858f ("xfs: reduce lock hold times in buffer writeback")
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_buf.c

@@ -1992,7 +1992,6 @@ xfs_buf_delwri_submit_buffers(
 	struct list_head	*wait_list)
 {
 	struct xfs_buf		*bp, *n;
-	LIST_HEAD		(submit_list);
 	int			pinned = 0;
 	struct blk_plug		plug;