go.net/ipv6: remove unused test code
go.net/ipv6: remove unused test code

R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/45630043

cesanta/goxnet,albertjin/golang-net,subuk/net,zabawaba99/net,kiip/net,DramaFever/golang.org-x-net,yangxianjun/net,dafyddcrosby/net,cmars/tools,raboof/net,veliamsli/net,gomini/net,muzining/net,dafyddcrosby/net,cmars/tools,mckinney-subgraph/net,muzining/net,DramaFever/golang.org-x-net,w4ngyi/net,gomini/net,raboof/net,chenzlabs/net,foreversmart/net,zabawaba99/net,kiip/net,mckinney-subgraph/net,flowlo/go-net,w4ngyi/net,cesanta/goxnet,chenzlabs/net,albertjin/golang-net,golang/net,yangxianjun/net,cmars/tools,veliamsli/net,cmars/tools,flowlo/go-net,subuk/net,foreversmart/net