Staging: benet: remove driver now that it is merged in drivers/net/

The benet driver is now in the proper place in drivers/net/benet, so we
can remove the staging version.

Acked-by: Sathya Perla <sathyap@serverengines.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>