Remove `ddclient_container_network` in favor of the role's configuration

On the role the value of `ddclient_container_network` is set to `ddclient_identifier`, which is set to `matrix-dynamic-dns` on the playbook.

Signed-off-by: Suguru Hirahara <did🔑z6MkvVZk1A3KBApWJXv2Ju4H14ErDfRGxh8zxdXSZ4vACDg5>

group_vars/matrix_servers

@@ -3354,8 +3354,6 @@ ddclient_identifier: matrix-dynamic-dns
 
 ddclient_docker_image_registry_prefix_upstream: "{{ matrix_container_global_registry_prefix_override if matrix_container_global_registry_prefix_override else ddclient_docker_image_registry_prefix_upstream_default }}"
 
-ddclient_container_network: matrix-dynamic-dns
-
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 # /matrix-dynamic-dns