net/http: fix incredibly racy TestTransportReading100Continue
net/http: fix incredibly racy TestTransportReading100Continue

Whoops. I'm surprised it even worked before. (Need two pipes,
not one.)

Also, remove the whole pipe registration business, since it
wasn't even required in the previous version. (I'd later fixed
it at the end of send100Response, but forgot to delete it)

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8191044

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