net: drop the unused helper skb_ext_get()

Such helper is currently unused, and skb extension users are
better off using skb_ext_add()/skb_ext_del(). So let's drop
it.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>