Remove matrix_ldap_registration_proxy_container_additional_networks_custom mention in docs

ldap-registration-proxy is already connected to the homeserver
container's network by default (via group vars), so there's no need for this.

docs/configuring-playbook-matrix-ldap-registration-proxy.md

@@ -30,9 +30,6 @@ matrix_ldap_registration_proxy_ldap_base_dn: "{{ matrix_synapse_ext_password_pro
 matrix_ldap_registration_proxy_ldap_user: "{{ matrix_synapse_ext_password_provider_ldap_bind_dn }}"
 matrix_ldap_registration_proxy_ldap_password: "{{ matrix_synapse_ext_password_provider_ldap_bind_password }}"
 
-matrix_ldap_registration_proxy_container_additional_networks_custom:
-  - "{{ matrix_synapse_container_network }}"
-
 matrix_ldap_registration_proxy_systemd_wanted_services_list_custom:
   - matrix-synapse.service
 ```