Failed conversions should be purged when converter plugins sortorder is changed

Scenario:

1. Unoconv converter is enabled (may be any other converter).
2. Students submit various files to their assignments.
3. Unovonv fails to convert files and leaves records in the mdl_file_conversion table with status = '-1'.
4. Admins configure and enable new file converter (may be LibreLambda or any other plugin).
5. New file coverter successfully converts all new files, but all failed pre-existing conversion remain unconverted.
6. Even if students re-submit same files they still fail to get converted.
7. Students can tweak and re-save their files (which changes file contenthash) so these new files get successfully converted (as there is no record with failed status in the mdl_file_conversion table).

Suggestion:

1. When a document converter is enabled/disabled all failed conversion should be deleted from the mdl_file_conversion table.