Destroy work items switch (permanent delete)

[muhahaaa]

Remove-ADOWorkItem.ps1 - Added the functionality to permanently delete work items via "-Destroy" switch parameter. Updated the docs to reflect this change.

Background: For data protection and privacy reasons we regularly have to delete work items permanently from our Azure DevOps project. That's why implemented the destroy switch.

[norschel]

@StartAutomating this pr is open since 2,5 years.