net/i40e: remove redundant number of packets check

net/i40e: remove redundant number of packets check

For i40e_xmit_pkts_vec_xx function, it checks nb_pkts to ensure nb_pkts
does not cross rs_thresh.

However, in i40e_xmit_fixed_burst_vec_xx function, this check will be
performed again. To improve code, delete this redundant check.

Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Qi Zhang <qi.z.zhang@intel.com>