firewalld: don't activate IPv6_rpfilter to be compatible with re6st

See https://firewalld.org/documentation/configuration/firewalld-conf.html#ipv6_rpfilter

When re6st is running with tunnels, the IPv6 packets can come in through
an interface and go out through another. This is expected.
1 job for galene-ptt
Status Job ID Name Coverage
  External
passed SlapOS.Theia.ResilienceTest-Master-ERP5