EDITED to change paths to /http/index.html for 1.4 firmware.

So you created a file at:

to serve your custom file instead of the Lantronix web configuration manager, but now want to delete it so that you can go back to the web configuration manager.

Assuming you're on Release 3 Alpha 2 or later, you can use the File System Web API to delete the file.

From the command line with curl, do this:
curl -s -u admin:PASSWORD -X DELETE
Or use your favorite REST API generator (like Postman for Chrome, my favorite!) to send the same thing. Here's a picture that shows you how:

Attached Files