netfilter: nftables: remove documentation on static functions

Since 4f16d25c ("netfilter: nftables: add nft_parse_register_load()
and use it") and 345023b0

 ("netfilter: nftables: add
nft_parse_register_store() and use it"), the following functions are not
exported symbols anymore:

- nft_parse_register()
- nft_validate_register_load()
- nft_validate_register_store()

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