UI: Rename 'remove' autotagger command to 'delete'

While importing my music collection in order to clean up its metadata, for some reason (possibly because of a config option, possibly because I organize my collection as flat folders of artists, not folders of albums), I kept getting prompted again and again about albums already being in the database; I usually hit 'keep' but was unsure what I was doing. I had read the [duplicates section of the manual](http://beets.readthedocs.org/en/v1.3.7/guides/tagger.html#duplicates) but it didn't really make the issues clear to me:

> Beets wants to keep you safe from duplicates, which can be a real pain, so you have three choices in this situation. You can skip importing the new music, choosing to keep the stuff you already have in your library; you can keep both the old and the new music; or you can remove the existing music and choose the new stuff. If you choose that last “trump” option, any duplicates will be removed from your library database—and, if the corresponding files are located inside of your beets library directory, the files themselves will be deleted as well.

I had specified in my config that

```
import:
resume: yes
write: yes
copy: no
languages: en
group_albums: yes
```

So I reasoned that the 'remove' probably meant something like beets was finding two albums of the same name but different MusicBrain unique IDs and hitting 'remove' meant I was reusing the original unique ID. Or something like that. It wouldn't mean that it was deleting the files because I had specifically told beet to leave my music in place with `copy: no` although it was allowed to write tags.

After a while I noticed that a bunch of my music files were gone. Apparently 'remove album' actually meant 'delete track'. Fortunately I had expected something like this to happen and had made a backup right before starting so I'm amused rather than angry, but I think maybe the manual could place more emphasis on the 'it will delete your files yo' and the autotager interface could explicitly call it 'delete'.

Start with the autotagger prompt and the manual's duplicates section linked in the issue. Rename the “remove” choice to “delete” and make the manual explicitly state that choosing it can delete music files; done when both interfaces use clear, consistent wording.

Written by the indexing model from the issue text.