Surveyor will list all the currently installed layouts.
You can't remove the built-in layouts. Any attempt to do so is likely to wreck the program and you'll have to reinstall it.
In TRS2004's Surveyor you can uninstall any layouts that you've created yourself. They are marked with a "c" (for "custom") and when one of them is selected a delete option appears.
In TRS2004's Surveyor you can't uninstall any layouts you've downloaded, but as others have said you can do this in the ever excellent Trainz Objectz.
Deleting a layout doesn't delete any of its dependencies, of which there may be hundreds. In Trainz Objectz there's an option to check for unused assets and delete them.
Sadly Trainz Objectz isn't compatible with TRS2006 or Trainz Classics/Trainz Railways/Trainz Railwayz so you'll have to do battle with the dreaded CMP. Good luck.
John