cifs: remove dead code

cifs provides two dummy functions 'sess_auth_lanman' and
'sess_auth_kerberos' for the case in which the respective
features are not defined. However, the caller is also under
an #ifdef, so we just get warnings about unused code:

fs/cifs/sess.c:1109:1: warning: 'sess_auth_kerberos' defined but not used [-Wunused-function]
 sess_auth_kerberos(struct sess_data *sess_data)

Removing the dead functions gets rid of the warnings without
any downsides that I can see.

(Yalin Wang reported the identical problem and fix so added him)

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
Signed-off-by: Steve French <smfrench@gmail.com>