Remove logic for restoring legacy JSON backups

- Protobuf backups have been around for 1.5 years now
- The ability to restore online-dependant data from JSON backups gets harder as time goes on and sources drift
- If users really need a way to restore them, they can use an older version of the app, or a separate tool for translating between the formats could be created

app/src/main/res/values/strings.xml

@@ -436,16 +436,13 @@
     <string name="pref_backup_service_category">Automatic backups</string>
     <string name="pref_backup_interval">Backup frequency</string>
     <string name="pref_backup_slots">Maximum backups</string>
-    <string name="source_not_found_name">Source not found: %1$s</string>
     <string name="tracker_not_logged_in">Not logged in: %1$s</string>
     <string name="backup_restore_invalid_uri">Error: empty URI</string>
     <string name="backup_created">Backup created</string>
     <string name="invalid_backup_file">Invalid backup file</string>
-    <string name="invalid_backup_file_missing_data">File is missing data.</string>
     <string name="invalid_backup_file_missing_manga">Backup does not contain any manga.</string>
     <string name="backup_restore_missing_sources">Missing sources:</string>
     <string name="backup_restore_missing_trackers">Trackers not logged into:</string>
-    <string name="backup_restore_content">Restore uses sources to fetch data, carrier costs may apply.\n\nMake sure you have installed all necessary extensions and are logged in to sources and tracking services before restoring.</string>
     <string name="backup_restore_content_full">Data from the backup file will be restored.\n\nYou will need to install any missing extensions and log in to tracking services afterwards to use them.</string>
     <string name="restore_completed">Restore completed</string>
     <string name="restore_duration">%02d min, %02d sec</string>