xtables: Drop xtables' family on demand feature

xtables: Drop xtables' family on demand feature

This conditional h->family assignment was added by commit 3f7877e6be987
("xtables-restore: add -4 and -6 support") with the intention to support
something like 'xtables-restore -6 <ip6tables.dump', i.e. having
family-agnostic commands which accept flags to set the family. Yet
commit be70918eab26e ("xtables: rename xt-multi binaries to -nft,
-legacy") removed support for such command names back in 2018 and nobody
has complained so far. Therefore drop this leftover as it makes
do_parse() more generic.

Signed-off-by: Phil Sutter <phil@nwl.cc>