s4-auth: Allow simple bind login of a user with an @ in the samAccountName

s4-auth: Allow simple bind login of a user with an @ in the samAccountName

LDAP Simple BIND authentications have already been mapped to a
DOMAIN\username pair and should not be mapped twice.

This appears to be a regression in 09e24ce40f89ac2f03d0c5fefa8b59f0d113fa6b
included in Samba 4.7.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13598

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Isaac Boukris <iboukris@samba.org>
Autobuild-User(master): Isaac Boukris <iboukris@samba.org>
Autobuild-Date(master): Fri Feb 14 17:13:33 UTC 2020 on sn-devel-184