http2: remove unused variable
http2: remove unused variable

Leftover from https://golang.org/cl/18932

Change-Id: I41c1adfdebd7dff0f75c2ba2b669f13b163984a2
Reviewed-on: https://go-review.googlesource.com/18936
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cesanta/goxnet,cmars/tools,cmars/tools,golang/net,cmars/tools,DramaFever/golang.org-x-net,albertjin/golang-net,albertjin/golang-net,cesanta/goxnet,cmars/tools,DramaFever/golang.org-x-net