dco: cleanup FreeBSD dco_do_read()

dco: cleanup FreeBSD dco_do_read()

Remove support for reading packets through the control interface.
FreeBSD no longer does this, so there's no point in keeping the code for it.

While here also check that we know what type of notification we're
getting. There's currently only one, but we should check anyway.

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221205164103.9190-5-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25616.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 235161cd2bcd5403c807e66432c421114c896b74)