It’s the reverse of Importing a .ADM into Group Policy.

1. Edit the Group Policy
2. Expand the Policies Folder
3. Select “Administrative Templates”
4. In the menu, select “Action”
5. Select “Add/Remove Templates…”
6. Highlight the template you want to remove
7. Click “Remove”, and then click “Close”
8. Exit the Group Policy Management Editor
9. Done!