netfilter: nf_tables: get rid of NFT_BASECHAIN_DISABLED

This flag was introduced to restore rulesets from the new netdev
family, but since 5ebe0b0e ("netfilter: nf_tables: destroy
basechain and rules on netdevice removal") the ruleset is released
once the netdev is gone.

This also removes nft_register_basechain() and
nft_unregister_basechain() since they have no clients anymore after
this rework.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>