You cannot remove the Adobe watermark from the tryout version exports - be it burn to disc product or export to file saved to the computer hard drive.
But, you can remove the Adobe watermark on a project file (project.prel) from the tryout. To do that open the project.prel in the purchased product.The classical answer is to then go to Timeline menu/Delete Rendered Files and click on Delete Rendered Files to do just that and get rid of the Adobe watermark. However, many of us have found the Adobe watermark automatically disappears from the tryout project.prel file when it is opened in the purchased product.
Please determine which of the above works for you and then let us know the outcome.