refactor: delete no longer needed code

[pavelbrm]

Summary

The main purpose of this PR is to delete the logic for handling the DELETE /v1/orders/{orderID} endpoint. This was an authenticated endpoint for internal use, and it has long ago been migrated away from to DELETE /v1/orders-new/{orderID}. The SKUs service is no longer responsible for cancelling a Stripe subscription in Stripe.

Additionally, the PR adds a missing test for repository.Order.GetByExternalID method.

It also deletes other code that has not been used for a while.

Type of Change

• Product feature
• Bug fix
• Performance improvement
• Refactor
• Other

Tested Environments

• Development
• Staging
• Production

Before Requesting Review

• Does your code build cleanly without any errors or warnings?
• Have you used auto closing keywords?
• Have you added tests for new functionality?
• Have validated query efficiency for new database queries?
• Have documented new functionality in README or in comments?
• Have you squashed all intermediate commits?
• Is there a clear title that explains what the PR does?
• Have you used intuitive function, variable and other naming?
• Have you requested security and/or privacy review if needed
• Have you performed a self review of this PR?

Manual Test Plan

[pavelbrm]

This does not do anything useful for Premium orders. The endpoint is already authenticated. The merchant is always the same, it's brave.com, and we set it ourselves. And it seems like caveats for Premium are not populated at all, so the check is useless at least, and it also wastes two queries to the database to fetch the order.