net: flow_offload: remove indirect flow_block declarations leftover

Remove function declarations that are not available in the tree anymore.

Fixes: 709ffbe1 ("net: remove indirect block netdev event registration")
Reported-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>