idna: Revert "use code generated by internal x/text package"

This reverts commit 67957fd.

Updates golang/go#18567

Change-Id: I4a9da509eb95949d2e3ab08763274abf6706f6f8
Reviewed-on: https://go-review.googlesource.com/35270
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benoit Sigoure <tsunanet@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: nigeltao