I'm still in Acrobat 8, but this seems to work. With your PDF open to the page you want, select the Touch-up Object tool. Right-click on the page and select Edit Page. That page should then open up in Illustrator, where you can edit/delete the mask. Save the changes in Illustrator, and it should update in Acrobat. I don't know what all could go wrong in this process, but it seems to work.