LDAP auth: remove config option 'ldap_load_groups'

The same effect can be achieved using the option 'ldap_groups_attribute' alone,
if it's default becomes unset instead of 'memberOf'

Benefit: one config option less to deal with.

While at it, also fix header level for 'ldap_user_attribute' in documentation.

radicale/config.py

@@ -247,12 +247,8 @@ DEFAULT_CONFIG_SCHEMA: types.CONFIG_SCHEMA = OrderedDict([
             "value": "",
             "help": "the attribute to be used as username after authentication",
             "type": str}),
-        ("ldap_load_groups", {
-            "value": "False",
-            "help": "load the ldap groups of the authenticated user",
-            "type": bool}),
         ("ldap_groups_attribute", {
-            "value": "memberOf",
+            "value": "",
             "help": "attribute to read the group memberships from",
             "type": str}),
         ("ldap_use_ssl", {