netfilter: nf_tables: remove nft_ctx_init_from_setattr()

Replace nft_ctx_init_from_setattr() by nft_table_lookup().

This patch also disentangles nf_tables_delset() where NFTA_SET_TABLE is
required while nft_ctx_init_from_setattr() allows it to be optional.

From the nf_tables_delset() path, this also allows to set up the context
structure when it is needed.

Removing this helper function saves us 14 LoC, so it is not helping to
consolidate code.

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