To delete mail template please check the below screens:
1) Access desired project that yours mail template is tied with (1):
%20-%20Accesing%20a%20desired%20project.png?version=1&modificationDate=1526379966615&cacheVersion=1&api=v2)
2) After clicking on "Add-ons" (1) go to Email Templates (2). Next, please click actions menu, as presented below (3) to be presented with a drop-down, where among other options, "Delete" (4) is present:
%20-%20deletion%20of%20a%20created%20email%20template.png?version=1&modificationDate=1526467962348&cacheVersion=1&api=v2)
3) Confirmation prompt will be present after that:
%20-%20Confirm%20delete.png?version=1&modificationDate=1526471158389&cacheVersion=1&api=v2&width=416&height=250)
Caution: after pressing "Delete" there's no way to revert the action.