runtime: remove unused variable Left over from cl/119490044.
runtime: remove unused variable
Left over from cl/119490044.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/125730043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go