[AC-2788] Remove unassigned items feature flag and logic

[fedemkr]

Type of change

• [ ] Bug fix
• [ ] New feature development
• [X] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
• [ ] Build/deploy pipeline (DevOps)
• [ ] Other

Objective

Remove unassigned items banner feature flag and related logic.

Code changes

• Reverted changes related to Unassigned items banner feature flag + logic.

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team