netfilter: Remove flowtable relics

NF_FLOW_TABLE_IPV4 and NF_FLOW_TABLE_IPV6 are invisble, selected by
nothing (so they can no longer be enabled), and their last real users
have been removed (nf_flow_table_ipv6.c is empty).

Clean up the leftovers.

Fixes: c42ba429 ("netfilter: flowtable: remove ipv4/ipv6 modules")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>