Remove the / -> : replacement from the File constructor

This was added in https://github.com/w3c/FileAPI/commit/5cd3dda4b181da227c78f7a1dabdd1be3ac60ecd, allegedly in response to https://www.w3.org/Bugs/Public/show_bug.cgi?id=23147. Neither Chrome nor Gecko seems to implement it.
