xenbus: drop useless LIST_HEAD in xenbus_write_watch() and xenbus_file_write()

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: 1107ba88 ("xen: add xenfs to allow usermode <-> Xen interaction")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>