"Remove new" option for duplicate resolution prompt in importer

When importing, I currently have 2 different cases mapped to a single outcome:
- beets cannot correctly tag a file -> skip it, fix later manually
- beets tells me "This item is already in the library!" and the new album is worse than the old one
- "Keep both" creates duplicates
- "Remove old" removes the better one
- "Skip it" mixes the new, bad one with the files I skipped to review them later. Since beets doesn't fix tags on skipped files, I have to remember externally which ones I'd like to delete and which ones to review.

I'd like to have a "Remove new" prompt in the "This item is already in the library!" prompt, so I can directly delete dupes I do not want (e.g. because of low quality).

Start by locating the importer entry point and the duplicate-resolution prompt that offers “Keep both,” “Remove old,” and “Skip it.” Trace how each choice handles the files, then add the requested “Remove new” behavior so the newly imported duplicate is deleted without affecting the existing library item.

Written by the indexing model from the issue text.