hv_netvsc: remove unused variable in netvsc_send()

With commit b56fc3c5 ("hv_netvsc: Fix a bug in netvsc_start_xmit()"),
skb variable is no longer used in netvsc_send. Remove variable and dead
code that depended on it.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>