There is no need to test __FreeBSD_version for features that have
been around for a long time now (7.1-ish or even earlier); assume
they are present. These includes MSI, TSO, LRO, VLAN, INTR_FILTERS,
FIRMWARE, etc.

Also, eliminate some dead code and clean up in other places as part
of this quick once-over.

MFC after: 1 week