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>