IB/ipoib: drop useless LIST_HEAD

Drop LIST_HEAD where the variable it declares is never used.

Commit 31c02e21 ("IPoIB: Avoid using stale last_send counter
when reaping AHs") removed the uses, but not the declaration.

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

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

Fixes: 31c02e21

 ("IPoIB: Avoid using stale last_send counter when reaping AHs")
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>