os: remove unused func itox

os: remove unused func itox

It was added by CL 288792 but was never used.

Change-Id: I0c21237c0c32ee5df2ebe99b6ecd8831f9c28649
Reviewed-on: https://go-review.googlesource.com/c/go/+/518316
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>