i40e: Drop unused completed stat

The Tx "completed" stat was part of the original rewrite for detecting
Tx hangs.  However some time ago in ixgbe I determined that we could
just use the packets stat instead.  Since then this stat was
removed from ixgbe and it serves no purpose in i40e so it can be
dropped.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>