I did think of doing that, but being very new to FVTT, and having spent a lot of hours importing and updating my campaign into FVTT prior to my first game using it this evening, I was hesitant to export and delete the world! Instead, I did in the end spend the hours moving every item into a folder, and then deleting the folder!
However, it’s really useful to know there isn’t currently a simple way of doing this that I had overlooked. Hopefully one day there will be a multi-select option for items in a list, or a way to clear down at the top level.
But given that a member of The Forge Staff has suggested exporting and reimporting the world, I have more confidence that doing so will not break anything, so I’ll do that in future!