Exclude deleted submissions from submissions property

[matthew-white]

Over the course of verifying v1.1.0, I soft-deleted a submission and noticed that the submissions property of the extended-metadata response for the form included the deleted submission. If you navigate to the form overview for that form, you see a submission count of 261:

https://test.central.getodk.org/#/projects/233/forms/all-widgets

However, once the submissions are fetched on the Submissions page, it turns out that there are only 260:

https://test.central.getodk.org/#/projects/233/forms/all-widgets/submissions

We don't provide a way to delete a submission over the API, so we might not need to change anything right now.

[matthew-white]

Closed by #331.