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

This reverts commit 67957fd0b1868d2b9f4fd2b3a3474c83bc3d49c3.

Updates golang/go#18567.

Change-Id: If4e07f09c9460fda4150201a11a8a3844067b824
Reviewed-on: https://go-review.googlesource.com/35370
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
7 files changed