@Anonymous Thanks for the confirmation.
Unfortunately, there is no PowerShell command that can help you to delete the message from service bus queue/subscription.
The workaround is leveraging the REST API in your PowerShell script to make the HTTP call to read and delete the message from dead letter queue.
Alternatively, you can also leverage the service bus SDK's, azure function/logic app to consume/delete the message from dead letter queue by writing your custom code.
Let me know if you have any queries or concerns.