Clean orphaned grade records in upgrade.php

[michaelkotlyar]

Relating to issue 813, we need to add a script that safely cleans all orphaned grade records (grade_grade and grade_item (and others?..)) relating to attendance module instances.

[danmarsden]

I've flagged this as an enhancement because the underlying issue is fixed and it would be nice to add a clean up task, however this isn't likely to be something I spend time on as a volunteer - happy to receive a pull request or funding to improve the attendance plugin ... or a WR if it's a Catalyst client ;-)