Remove unused confirm_bulk_delete functionality

[legoktm]

Status

Ready for review

Description of Changes

This page was originally used as a confirmation interstitial when deleting
files or messages, but in 1e64f36 (#2946) it was replaced with a CSS-only
deletion confirmation on the same page, so users would never hit the
separate confirmation page.

Remove the tests that were still calling this endpoint, the HTML template
and code that was checking for the "confirm_delete" action.

I noticed this when investigating a translator comment about the "Permanently Delete Files" text and was unable to figure out how to take a screenshot of it. We probably don't need to get this into 2.2.0, but maybe we can mark these strings as not requiring translation in the meantime.

Testing

• Verify that you still see a confirmation dialog in the JI when deleting files/messages

Deployment

Any special considerations for deployment? No

Checklist

• Linting (make lint) and tests (make test) pass in the development container
• These changes do not require documentation

[cfm]

I'll be happy to review this in this sprint, @legoktm. I developed the same suspicion while working on #6165 but didn't have time to prove that this view and logic could indeed be removed. :-)

[cfm]

Confirmed all deletion flows appear and function as expected:

1. /: select one or more sources → Delete → Files and Messages
2. /: select one or more sources → Delete → Source Accounts →Yes, Delete Selected Source Accounts
3. /col/<filesystem_id>: select one or more entries → Delete Selected → DELETE

Nothing like a good all-deletion pull request. :-)