net: remove indirect block netdev event registration

Drivers do not register to netdev events to set up indirect blocks
anymore. Remove __flow_indr_block_cb_register() and
__flow_indr_block_cb_unregister().

The frontends set up the callbacks through flow_indr_dev_setup_block()

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>