How to delete previous and useless form data from "export form" menu?

I have used commcare 1 year and there become too much data at “export form”.

How to delete previous and useless form data from “export form” menu?

(I already know how to delete custom forms surely that is clicking the
delete icon of exported custom form. That is easy.)

The problem is how to delete useless forms which are shown in "form export"
except exported custom form.

Could you please help me?

Thanks a lot.