The abstraction was useful for handling 2 systems, but it's no longer needed. Cleaning it up will make migrating to domain models easier down the line.
(cherry picked from commit a2bb81b7db457c3927eaa1f03416a9b07985842b)
# Conflicts:
# app/src/main/java/eu/kanade/tachiyomi/data/backup/BackupManager.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/BackupRestorer.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/Backup.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/BackupFlatMetadata.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/BackupMergedMangaReference.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/BackupSavedSearch.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/metadata/BackupSearchMetadata.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/metadata/BackupSearchTag.kt
# app/src/main/java/eu/kanade/tachiyomi/data/backup/models/metadata/BackupSearchTitle.kt
6f6a9b677c