build: remove special definition of Runemax on Plan 9
build: remove special definition of Runemax on Plan 9

The Bell Labs distribution now supports 21-bit runes.

R=golang-dev, akumar
CC=golang-dev
https://codereview.appspot.com/9249045

firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old