To delete mail template please check the below screens:

1) Access desired project that yours mail template is tied with (1):


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:


3) Confirmation prompt will be present after that:

(warning) Caution: after pressing "Delete" there's no way to reverse the action.