net: don't return a nested error when happy eyeballs dialing
net: don't return a nested error when happy eyeballs dialing

Also removes an unused variable.

Fixes issue 6795.

R=adg, dave, bradfitz, gobot
CC=golang-dev
https://codereview.appspot.com/29440043

bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc