hyperv/hn: Remove assign-only struct field

MFC after:	1 week
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D7554

sys/dev/hyperv/netvsc/hv_net_vsc.c

@@ -88,7 +88,6 @@ hv_nv_alloc_net_device(struct hn_softc *sc)
 	net_dev = malloc(sizeof(netvsc_dev), M_NETVSC, M_WAITOK | M_ZERO);
 
 	net_dev->sc = sc;
-	net_dev->destroy = FALSE;
 	sc->net_dev = net_dev;
 
 	return (net_dev);
@@ -734,9 +733,6 @@ hv_nv_on_device_remove(struct hn_softc *sc, boolean_t destroy_channel)
 {
 	netvsc_dev *net_dev = sc->net_dev;;
 	
-	/* Stop outbound traffic ie sends and receives completions */
-	net_dev->destroy = TRUE;
-
 	hv_nv_disconnect_from_vsp(net_dev);
 
 	/* At this point, no one should be accessing net_dev except in here */

sys/dev/hyperv/netvsc/hv_net_vsc.h

@@ -236,8 +236,6 @@ typedef struct netvsc_dev_ {
 	/* Holds rndis device info */
 	void					*extension;
 
-	uint8_t					destroy;
-
 	uint32_t                                num_channel;
 
 	struct hyperv_dma			rxbuf_dma;