Remove an unused macro we originally got from BSD/OS.
Move an assertion to the right spot; only bus_dmamap_load_mbuf(9)
requires a pkthdr being present but that's not the case for either
_bus_dmamap_load_mbuf_sg() or bus_dmamap_load_mbuf_sg(9).

Reported by:	sbruno
MFC after:	1 week