nfsd: discard nfsd_file_get_local()

This interface was deprecated by commit e6f7e148 ("nfs_localio:
simplify interface to nfsd for getting nfsd_file") and is now
unused. So let's remove it.

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>