if_epair: delete mbuf tags

Remove all (non-persistent) tags when we transmit a packet. Real network
interfaces do not carry any tags either, and leaving tags attached can
produce unexpected results.

Reviewed by: bz, glebius
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32663