cifs: remove the cifs_ses->flags field

This field is completely unused:

CIFS_SES_W9X is completely unused. CIFS_SES_LANMAN and CIFS_SES_OS2
are set but never checked. CIFS_SES_NT4 is checked, but never set.

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