CI: Remove the unused backport workflow

[seisman]

Description of proposed changes

The backport workflow can backport changes in a merged PR into a specific branch. It's frequently used when we were at GMT 6.1, but we haven't used it for > 3 years.

I think it's OK to delete it. If we need backport in the future, we can add it back or use other alternative tools like https://github.com/sqren/backport-github-action and https://github.com/sqren/backport.

Fixes #

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.