net: dsa: Remove unused __DSA_SKB_CB macro

The struct __dsa_skb_cb is supposed to span the entire 48-byte skb
control block, while the struct dsa_skb_cb only the portion of it which
is used by the DSA core (the rest is available as private data to
drivers).

The DSA_SKB_CB and __DSA_SKB_CB helpers are supposed to help retrieve
this pointer based on a skb, but it turns out there is nobody directly
interested in the struct __dsa_skb_cb in the kernel. So remove it.
Signed-off-by: NVladimir Oltean <olteanv@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -94,8 +94,6 @@ struct __dsa_skb_cb {
 	u8 priv[48 - sizeof(struct dsa_skb_cb)];
 };
 
-#define __DSA_SKB_CB(skb) ((struct __dsa_skb_cb *)((skb)->cb))
-
 #define DSA_SKB_CB(skb) ((struct dsa_skb_cb *)((skb)->cb))
 
 #define DSA_SKB_CB_PRIV(skb)			\