Remove max automatic backups setting

We just always create up to 4 now to simplify it, given the addition of syncing
is going to make this section pretty busy.

(cherry picked from commit e3b70ca08d1c5a09a448891ff66102fd696d7098)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/data/backup/BackupCreator.kt

domain/src/main/java/tachiyomi/domain/backup/service/BackupPreferences.kt

@@ -11,8 +11,6 @@ class BackupPreferences(
 
     fun backupsDirectory() = preferenceStore.getString("backup_directory", folderProvider.path())
 
-    fun numberOfBackups() = preferenceStore.getInt("backup_slots", 2)
-
     fun backupInterval() = preferenceStore.getInt("backup_interval", 12)
 
     fun lastAutoBackupTimestamp() = preferenceStore.getLong(Preference.appStateKey("last_auto_backup_timestamp"), 0L)