nfsd: drop useless LIST_HEAD

Drop LIST_HEAD where the variable it declares is never used.

This was introduced in c5c707f9 ("nfsd: implement pNFS
layout recalls"), but was not used even in that commit.

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

)

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

Fixes: c5c707f9 ("nfsd: implement pNFS layout recalls")
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>