fix: account for license source records during license retirement

[iloveagent57]

ENT-8172 | Delete related SubscriptionLicenseSource records when licenses are retired or expired. Without this change, re-assignments of retired licenses (remember that we throw licenses "back into the pool" on revocation, retirement, or expiration) causes an integrity error on the license source table.

Consequences

1. Licenses that are retired due to edX user retirement (like PII-scrubbing) will lose their source identifiers - we won't have a way to re-associate those to the license or associated enrollments. But I think this is ok because of what it means to retire a user.
2. Licenses that expire lose their associated source identifier. I think that's fine, b/c expired licenses are those that haven't been activated in the 90 day activation window, so there's no enrollments associated with them.

Link to the associated ticket: https://openedx.atlassian.net/browse/ENT-8172

Testing considerations

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

[adamstankiewicz]

LGTM!