Revert "netfilter: flowtable: teardown flow if cached mtu is stale"

This reverts commit b8baac3b.

IPv4 packets with no DF flag set on result in frequent flow entry
teardown cycles, this is visible in the network topology that is used in
the nft_flowtable.sh test.

nft_flowtable.sh test ocassionally fails reporting that the dscp_fwd
test sees no packets going through the flowtable path.

Fixes: b8baac3b ("netfilter: flowtable: teardown flow if cached mtu is stale")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>