I would assume there is an add-in somewhere that finds unused View Templates and allows you to delete them, but it must be done manually with vanilla Revit.
Deleting a View Template - WikiHelp
View Filters & View Templates - Revit
Revit Beginners: Additive Views vs. Subtractive Views
Purging Unused Views – Sheets | BD Mackey Consulting | The Revit Geek Blog