Remove the ability for the superuser to delete users

[cielf]

Summary

Remove the button for deleting users from the admin's user list

Why?

We don't need it, there is potential confusion, and if it is used on a partner who has ever made a requests, we get a 500.

Details

Remove the button for deleting users from the admin user index (N.B. not the button for deleting roles on the individual view)
To see this, sign in as superadmin@example.com, click Users in the left-hand menu, then All Users

Criteria for completion

• the button is removed
• tests modified to reflect the new behaviour
• tests pass
• screenshots in the PR are a nice-to-have.

[noah-puckett]

Hello @cielf, could I try taking this on?

[cielf]

Go for it!