If you no longer need a content template, you can delete it. Once deleted, content for that type will no longer use the custom layout and styling you configured and will instead fall back to the default display.
Instructions
Sign in to your site.
Navigate to and click Site Tools on the toolbar:

Click Content Templates:

Locate the content template you want to delete. Click the Ellipsis for the template:

Select Delete:

Click the Delete button to confirm that you want to delete the content template:

Important Note:
The content template will be permanently deleted.