Remove `beets.util.confit`

[bal-e]

The commit to deprecate confit was made on June 1, 2019. It's been 5 years! It is now removed entirely.

To Do

• [x] ~Documentation~
• [ ] Changelog
• [x] ~Tests~

[github-actions[bot]]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.