mac80211: remove superfluous NULL check

At the place where this code lives now, the skb can never be
NULL, so we can remove the pointless NULL check.

It seems to exist because this code was moved around a few times
and originally came from a place where it could in fact be NULL.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3583,7 +3583,7 @@ begin:
 			skb_queue_splice_tail(&tx.skbs, &txqi->frags);
 	}
 
-	if (skb && skb_has_frag_list(skb) &&
+	if (skb_has_frag_list(skb) &&
 	    !ieee80211_hw_check(&local->hw, TX_FRAG_LIST)) {
 		if (skb_linearize(skb)) {
 			ieee80211_free_txskb(&local->hw, skb);