e1000e: remove redundant if clause from PTP work

tx_hwtstamp_skb is always set before work is scheduled,
work is cancelled before tx_hwtstamp_skb is set to NULL.
PTP work cannot ever see tx_hwtstamp_skb set to NULL.
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -1148,9 +1148,6 @@ static void e1000e_tx_hwtstamp_work(struct work_struct *work)
 						     tx_hwtstamp_work);
 	struct e1000_hw *hw = &adapter->hw;
 
-	if (!adapter->tx_hwtstamp_skb)
-		return;
-
 	if (er32(TSYNCTXCTL) & E1000_TSYNCTXCTL_VALID) {
 		struct skb_shared_hwtstamps shhwtstamps;
 		u64 txstmp;