e1000: remove unused ifp backpointer

The ifp (struct ifnet) backpointer in the e1000 private ifnet
data is not used anymore since the iflib transition.
Remove it so that developers are not tempted to use it and
get a NULL pointer dereference.

Reviewed by:	markj, kbowling, erj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D33157

sys/dev/e1000/if_em.h

@@ -439,7 +439,6 @@ struct em_rx_queue {
 
 /* Our softc structure */
 struct e1000_softc {
-	struct ifnet 		*ifp;
 	struct e1000_hw		hw;
 
 	if_softc_ctx_t		shared;