ifconfig.8: remove accept_rev_ethip_ver and send_rev_ethip_ver

It is removed long ago [1] and unavailable on FreeBSD 11 and later,
but the corresponding entries in the man page were not removed
accordingly.

[1] https://cgit.freebsd.org/src/commit/?id=b1c250ff3ff6af36e95f5cf910973133c3445704

MFH after: 3 days
Reviewed by: mhorne
Approved by: mhorne
Differential Revision: https://reviews.freebsd.org/D51261