If you were in the "Email marketing" module when you created the UI element, you must have selected a different module when you created it. It would have had to be a module that was is in custom development mode.
To delete the UI element, navigate to it through the module you selected. For example, if you selected "Mike's Test Module", navigate to
Modules > Mike's Test Module > User Interface
Modules > Email Marketing > User Interface
After that, you should be able to select the element in the tree, and then click the delete button.