net/benet: Remove interface type

The interface type, which is being traced by "struct be_adapter::
if_type", isn't used currently. So we can remove that safely
according to Sathya's comments.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>