confdb: Remove CONFDB_DOMAIN_LEGACY_PASS
As CONFDB_DOMAIN_LEGACY_PASS is legacy parameter that is rooted in
pre-1.0 SSSD, let's just nuke it now as we're already removing other
legacy code.
Resolves:
https://pagure.io/SSSD/sssd/issue/3530
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>