net/http/httputil: tell people not to use ClientConn and ServerConn
A very smart developer at Gophercon just asked me to help debug
a problem and I was horrified to learn that he was using httputil's
ClientConn. I forgot ClientConn and ServerConn were even included
in Go 1! They should've been deleted.
Scare people away from using them. The net/http package does
not use them and they're unused, unmaintained and untouched in
4+ years.
LGTM=r
R=r, adg
CC=golang-codereviews
https://golang.org/cl/
92790043