feat(subsonic)!: drop support for guessed artist covers in filesystem

it doesn't make sense with multi-artist support anymore

gonic will use lastfm or an album cover instead

Release-As: 0.16.0

db/model.go

@@ -49,7 +49,6 @@ type Artist struct {
 	NameUDec      string   `sql:"default: null"`
 	Albums        []*Album `gorm:"many2many:album_artists"`
 	AlbumCount    int      `sql:"-"`
-	Cover         string   `sql:"default: null"`
 	ArtistStar    *ArtistStar
 	ArtistRating  *ArtistRating
 	AverageRating float64 `sql:"default: null"`