Fix CleanupGroupImportStatesWithNullUserId post deploy migration

What does this MR do?

Fix post deployment migration for scenarios that failed on production gitlab-com/gl-infra/production#2731 (closed)

Data migration failed when calling group.parent with Invalid single-table inheritance type: Group is not a subclass of CleanupGroupImportStatesWithNullUserId::Namespace.

This MR removes redefined STI parent association and instead redefines parent method instead, to do a simple group look up by id, avoiding STI altogether.

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done