if_ovpn: Remove a no-op function

ovpn used to maintain one socket ref per peer. This scheme was changed
in commit 3acf7e0da487 ("if_ovpn: avoid LOR between ovpn and UDP
locks"), which turned ovpn_rele_so() in a no-op. Just remove the whole
function and a related helper, as the remaining assertion there isn't
useful.

No functional change intended.

Reviewed by: kp
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51497