I'd guess that it's not implemented yet. Lots of things aren't 100% done on the Espruino.
What's your use case, if you don't mind me asking? SD cards are just so massive compared to the amounts of data the Espruino works with (5 orders of magnitude larger), I have to wonder what you're doing where you'd have a desire to delete files.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I'd guess that it's not implemented yet. Lots of things aren't 100% done on the Espruino.
What's your use case, if you don't mind me asking? SD cards are just so massive compared to the amounts of data the Espruino works with (5 orders of magnitude larger), I have to wonder what you're doing where you'd have a desire to delete files.