runtime: remove unused var
runtime: remove unused var

LGTM=bradfitz
R=daniel.morsing, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/130500044

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