nfsd: Drop LIST_HEAD where the variable it declares is never used.

The declarations were introduced with the file, but the declared
variables were not used.

Fixes: 65294c1f ("nfsd: add a new struct file caching facility to nfsd")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>