Remove SOA maximum parameters from DNS zone.

There's no such thing as "maximum" in SOA record RDATA format
according to RFC 1035 and there's also no such attribute in
the schema.

Fix #788

ipalib/plugins/dns.py

@@ -198,11 +198,6 @@ class dnszone(LDAPObject):
             default=3600,
             autofill=True,
         ),
-        Int('idnssoamaximum?',
-            cli_name='maximum',
-            label=_('SOA maximum'),
-            doc=_('SOA record maximum value'),
-        ),
         Int('dnsttl?',
             cli_name='ttl',
             label=_('SOA time to live'),