netfilter: nf_tables: remove NFT_CT_TIMEOUT

Never used anywhere in the code.

Fixes: 7e0b2b57

 ("netfilter: nft_ct: add ct timeout support")
Reported-by: Stéphane Veyret <sveyret@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>