sockets: remove compat shim for divert(4)

All known software in ports had been addressed three years ago and the
shim stays in stable/14 and stable/15 for another couple years with its
printf(), so all ourliers are expected to conform before 16.0-RELEASE.
See 8624f4347e8133911b0554e816f6bedb56dc5fb3 for details.