net/packet: Remove unused macro BLOCK_PRIV

BLOCK_PRIV is never used after it was introduced.
So better to remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>