fix: drop superadmin entries from assignment list endpoint

[mariajgrimaldi]

Description

Staff/superuser entries were fetched separately from role assignments in AssignmentsAPIView, so org/scope/role filters did not apply to them. This PR removes them from the list endpoint entirely for now. The single-user endpoint (TeamMemberAssignmentsAPIView) still returns them.

Follow-up: #347

Testing Instructions

1. Log in as a staff/superuser.
2. Call GET /api/authz/v1/assignments/ and verify no django.staff or django.superuser entries appear.
3. Apply any org, scope, or role filter and verify the count reflects only role assignments.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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