netfilter: nf_tables: rename nf_table_delrule_by_chain()

For the sake of homogenize the function naming scheme, let's rename
nf_table_delrule_by_chain() to nft_delrule_by_chain().

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>