if_epair: also remove vlan metadata from mbufs

We already remove mbuf tags from packets transitting an if_epair, but we
didn't remove vlan metadata.
In certain configurations this could lead to unexpected vlan tags
turning up on the rx side.

PR: 270736
Reviewed by: markj
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39482