samba-dc: Switch away from BIND9_DLZ

The BIND9_DLZ plugin turned out to be pretty flaky. It craps out
whenever `named` is reloaded, which seems to happen occasionally for
reasons I cannot identify. Combined with the weird SELinux issues, and
the fact that upstream recommends against it anyway, I decided to just
use the built-in DNS server in Samba.

group_vars/samba-dc.yml

@@ -1,14 +0,0 @@
-named_keytab: /var/lib/samba/bind-dns/dns.keytab
-named_global_include:
-- /var/lib/samba/bind-dns/named.conf
-samba_dc_dns_backend: BIND9_DLZ
-named_listen:
-- addresses:
-  - any
-named_listen_v6:
-- addresses:
-  - any
-named_allow_query:
-- any
-named_allow_update:
-- any