build: remove rm dreg
build: remove rm dreg

Undo CL 4254056 now that enough time has elapsed
that people with old trees have all built and removed
those files (which are no longer generated in that location).

Fixes issue 1581.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4802059

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