pfctl: Do not warn if there is no Ethernet anchor

Avoid emitting a warning if there is no Ethernet anchor. If the anchor
--regardless of its type-- is nonexistent, should be caught earlier.

This can be misleading when there is a layer 3 anchor but not an
Ethernet anchor, giving the user the impression that there is no layer 3
anchor with that name.

PR: 280516
Approved by: kp
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D53360