net: dsa: Remove the now unused DSA_SKB_CB_COPY() macro

It's best to not expose this, due to the performance hit it may cause
when calling it.

Fixes: b68b0dd0 ("net: dsa: Keep private info in the skb->cb")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -99,9 +99,6 @@ struct __dsa_skb_cb {
 
 #define DSA_SKB_CB(skb) ((struct dsa_skb_cb *)((skb)->cb))
 
-#define DSA_SKB_CB_COPY(nskb, skb)		\
-	{ *__DSA_SKB_CB(nskb) = *__DSA_SKB_CB(skb); }
-
 #define DSA_SKB_CB_PRIV(skb)			\
 	((void *)(skb)->cb + offsetof(struct __dsa_skb_cb, priv))