Replaced by Ctxt.ByteOrder, which uses the standard binary.ByteOrder type.

Change-Id: I06cec0674c153a9ad75ff937f7eb934891effd0b
Reviewed-on: https://go-review.googlesource.com/6450Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Rob Pike <r@golang.org>