internal/socket, route: drop support for pre-1.12 direct syscalls on darwin

Go 1.12 and newer are using libSystem for syscalls on darwin.
Go 1.11 is the last release to use direct syscalls. It hasn't been
supported since the release of Go 1.13 in September 2019, so drop
support for direct syscalls on darwin from golang.org/x/net.

Change-Id: Ib53d64e8a6d12ad702951f8efc123ce92c01c4d9
Reviewed-on: https://go-review.googlesource.com/c/net/+/257057
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser