Next Gen-Not able to remove data from Assigned To field in helpdesk request

Summary:

Scenario :Once a request has been created , it has been assigned into queue.
Then an agent has assinged to himself using Assigned to field.
Now Agent decides to assign it to another queue and want to remove Assigned to field.
System is not allowing to delete/remove/empty the Assigned to field.

Can some one please assist as this is required for us.

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