ipv6: cleanup: remove unneeded null check

We dereference "sk" unconditionally elsewhere in the function.  

This was left over from:  b30bd282

 "ip6_xmit: remove unnecessary NULL
ptr check".  According to that commit message, "the sk argument to 
ip6_xmit is never NULL nowadays since the skb->priority assigment 
expects a valid socket."

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>