if an activity, event or deal is done and dusted it has been completed successfully and none of it is left to be done.

A term used to reference cannabis/marijuana/weed.

Well, thinking about this logically, projects and tasks cause a mess and/or the actual project has minor details to perfect it. When you're finished, you put away your tools and then dot your i's & cross your t's. When something is "done and dusted", it's not merely created or accomplished, it's also polished and cleaned up after. Nothing else is needed, so it can be considered "case closed".