Revert "if_ovpn: allow peer lookup by vpn4/vpn6 address"

This reverts commit 92f0cf77db18502cac9a731cd2f6e8f3cc8a9369.

This change was incorrect, at least because it uses ovpn_kpeer's tree
for multipbe RB_TREEs.

This is a performance change, not a functional one, so we can revert
this until it can be fixed.

Reported by: Gert Doering <gert@greenie.muc.de>
Sponsored by: Rubicon Communications, LLC ("Netgate")