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

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

This reverts commit b8baac3b9c5cc4b261454ff87d75ae8306016ffd.

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: b8baac3b9c5c ("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>