It is already inside UE4 Editor, although it is not exactly one button ‘cleanup my assets, please’
- Create a new project (this is supposed to be the clean project with all needed asset)
- Inside UE4 Editor, working with existing project (which has many unneeded asset), select a level and then right click, Asset Action -> Migrate.
- Migrate to the new project just created (select the Content folder).
- The migration will copy all referenced asset to the new project
- Repeat for all levels.