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: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>