rabbit_ldap_seed: Remove 'memberOf' annotation for "peter" seed user

In openldap 2.5, `memberOf` seems to be a read-only annotation. Instead
the LDAPWiki recommends adding the user's name to the member attribute
on the relevant group objects:
<https://ldapwiki.com/wiki/Wiki.jsp?page=MemberOf>

We already do that when setting up the group objects, so this annotation
is safe to remove.

deps/rabbitmq_auth_backend_ldap/test/rabbit_ldap_seed.erl

@@ -158,10 +158,7 @@ peter() ->
                        "organizationalPerson",
                        "person"]},
       {"loginShell", ["/bin/bash"]},
-      {"userPassword", ["password"]},
-      {"memberOf", ["cn=wheel,ou=groups,dc=rabbitmq,dc=com",
-                    "cn=staff,ou=groups,dc=rabbitmq,dc=com",
-                    "cn=people,ou=groups,dc=rabbitmq,dc=com"]}]}.
+      {"userPassword", ["password"]}]}.
 
 carol() ->
     {"uid=carol,ou=people,dc=rabbitmq,dc=com",