Revert "net, internal/syscall/unix: add SocketConn, SocketPacketConn"

Revert "net, internal/syscall/unix: add SocketConn, SocketPacketConn"

This reverts commit 6f7961da28232c609f7c51b3bed7f15db7dd33e1.

Russ suggests changing the frozon syscall package and obviously it's a
better solution. Perhaps he will also let me know the way how to get the
project owners to agree later.

Fixes #11492.

Change-Id: I98f9f366b72b85db54b4acfc3a604b62fb6d783c
Reviewed-on: https://go-review.googlesource.com/11854
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>