IfAPI: Retire if_etherbpfmtap() and if_bpfmtap()

Summary:
These came in the original DrvAPI commits in 2014, and are obsoleted by
bpf_mtap_if() and ether_bpf_mtap_if(). The _if suffix, rather than
prefix, conveys that it's operating on the bpf of the interface, instead
than the interface itself.

Reviewed by: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D41146