[AC-2520] Remove Unassigned Items Banner (#10042)
* chore: remove UnassignedItemsBanner feature flag, refs AC-2520
* chore: remove unassignedItemsBanner from web-header component, refs AC-2520
* chore: delete unassigned items banner service/api/spec, refs AC-2520
* chore: remove unassigned items banner messages (web), refs AC-2520
* chore: remove unassigned items banner messages (browser), refs AC-2520
* chore: remove unassigned items banner code from current tab (browser), refs AC-2520
* chore: remove state definition for unassigned items banner, refs AC-2520
* chore: revert state-definition removal, refs AC-2520