SharePoint Server: Unlink Nintex form from your list using Powershell

*Disclaimer
Bear in mind that the solution presented below is neither Microsoft- nor Nintex-approved. It worked for me in my particular scenario and is likely to work for you, but it may not work in all scenarios and may have flaws or cause potential issues in other scenarios/environments which did not arise in my environment and scenario. *

Scenario
You have Nintex forms associated with your SharePoint lists. You want to remove Nintex solution from the site/farm.

Find the forms

Remove the forms