Delete Unwanted Projects
You would be able to delete projects in your project file if they are un wanted.
David Simons (Adobe) commented
Can you describe more about what you'd like to do? You can delete anything you want from the project panel. Any referenced source files will remain in the project folder on disk in case you hit undo.
If you know you don't need any undo history, you can use File > Save Project As to save a version of the project will all those old files removed.