NFSv4/pnfs: Remove redundant list check

pnfs_layout_free_bulk_destroy_list() already checks for whether the list
is empty or not.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>