src: Remove expire information from list stateless ruleset.

src: Remove expire information from list stateless ruleset.

As expires is stateful information. This patch removes expire
information from list stateless ruleset. With nft -s option, the
ruleset will be as following.

table ip firewall {
        set host {
                type ipv4_addr
                flags timeout
                elements = { 10.0.0.2 timeout 10m }
        }
}

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>