Allow deletion of unenrolled student's progress

[eri-trabiccolo]

Description

Fixes #173

How has this been tested?

manually and with new unit tests

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.

[eri-trabiccolo]

@thomasplevy sorry, changes require to be approved...