Getting Rid of It
There is nothing magical about an Xcode project. It’s just a directory on your hard drive. If you don’t want it any more, close the project, select its enclosing folder in the Finder, and drag it to the Trash. It’s gone. It won’t even show up in the Recents list in the Welcome to Xcode window, or in the File→Open Recent menu.
Okay, yes, the build products of the project will still stick around in a warren of directories inside ~/Library/Developer/Xcode/DerivedData. They aren’t many or large in this case, but there’s a principle involved.
If you want them gone, the best way is to close the project window, open the Organizer window (Window→Organizer, 2), select the Projects panel, select the “Hello World” project, press Delete, and confirm the deletion in the ensuing alert sheet. All trace of the build products is gone.