cifs: remove useless memset in LANMAN auth code

It turns out that CIFS_SESS_KEY_SIZE == CIFS_ENCPWD_SIZE, so this
memset doesn't do anything useful.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>