This partially reverts commit 3bdbb5df.
The latest CL of js/wasm's file system support does not use
file descriptor mapping any more.

Change-Id: Iaec9c84b392366282cddc69acc75c8a3eb556824
Reviewed-on: https://go-review.googlesource.com/114195Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>