ixgbe: Drop unnecessary call to rcu_barrier

The ixgbe driver hasn't used call_rcu to free the rings for some time now.
Since that is the case the call to rcu_barrier can be dropped since calls
to kfree_rcu don't require it.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>