[IPSEC]: Kill duplicate xfrm_policy_flush prototype

For five years we had two xfrm_policy_flush prototypes and every time that
function's signature changed people have been diligently updating both of
them without noticing :)

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>