net: ibmvnic: Remove unused net_stats member from struct ibmvnic_adapter

The ibmvnic driver keeps its statistics in net_device->stats, so the
net_stats member in struct ibmvnic_adapter is unused. Remove it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>