trove-dashboard still uses horizon create_instance..SetNetwork class code which is dropped in horizon

horizon dropped create_instance [1] code in an effort to remove Django-based implementation of Launch instance [2] but trove-dashboard still imports that code[1] which needs to be fixed.

[1] https://github.com/openstack/trove-dashboard/blob/master/trove_dashboard/content/databases/workflows/create_instance.py#L27
[2] https://review.opendev.org/c/openstack/horizon/+/816303/9/openstack_dashboard/dashboards/project/instances/workflows/create_instance.py