Purging View Templates ...

... is not easy.  Revit prefers that you don't clog up your drawing with View Templates rather than trying to clean them out later.  You can use various methods to transfer just one Template from project to project, such as this example.

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.

Further reading:
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