Is there a way of deleting a scale from a page (i.e. have a 'Set Scale' option to 'Scale Not Set')?
Not that I have found - I wouldn't mind one either - I made the mistake of adding a scale of my cover sheet, and it's an isometric view 😜
I think the best you can do for now is set the scale as 1:1
There is a way if you don't have any markups on the PDF and just want to remove the scale. Go to File > Create > From File. The only caveat, is that if you have any markups, you need to export them first - otherwise, they will become part of the PDF content.