rollouts: delete old root/reposync objects when synctemplate type changes

[natasha41575]

This is a followup item for https://github.com/GoogleContainerTools/kpt/pull/3850, which adds RepoSync support to the rollouts api.

However, with that PR, changing the syncTemplate type from rootSync to repoSync (or the other way around) will result in the new sync object being created in the target repo, but the controller does not currently clean up the old sync object. The task here is to ensure that the controller cleans up the old sync object.

[droot]

Assigning a lower priority since this is an edge case and very unlikely to occur.