netfilter: flowtable: Remove the empty file

CONFIG_NF_FLOW_TABLE_IPV4 is already removed and the real user is also
removed(nf_flow_table_ipv4.c is empty).

Fixes: c42ba429 ("netfilter: flowtable: remove ipv4/ipv6 modules")
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>