if_ovpn: remove OVPN_SEND_PKT

OpenVPN userspace no longer uses the ioctl interface to send control
packets. It instead uses the socket directly.
The use of OVPN_SEND_PKT was never released, so we can remove this
without worrying about compatibility.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37602