i40e/i40evf: remove time_stamp member

The driver doesn't use the time_stamp member to determine if there is a
tx_hang any more. There really isn't any point to the variable at all
so just remove it. It was left over from a previous tx_hang design.

Change-ID: I4c814827e1bcb46e45118fe37acdcfa814fb62a0
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>