intel/i40e: delete if NULL check before dev_kfree_skb

dev_kfree_skb check if the input parameter NULL and do the right
thing, there is no need to check again.
This change is to cleanup the code a bit.
Signed-off-by: Bernard Zhao <zhaojunkui2008@126.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -1483,10 +1483,8 @@ void i40e_clean_rx_ring(struct i40e_ring *rx_ring)
 	if (!rx_ring->rx_bi)
 		return;
 
-	if (rx_ring->skb) {
-		dev_kfree_skb(rx_ring->skb);
-		rx_ring->skb = NULL;
-	}
+	dev_kfree_skb(rx_ring->skb);
+	rx_ring->skb = NULL;
 
 	if (rx_ring->xsk_pool) {
 		i40e_xsk_clean_rx_ring(rx_ring);