netfilter: nf_tables: remove nft_meta_target

In e035b77a ("netfilter: nf_tables: nft_meta module get/set ops"),
we got the meta target merged into the existing meta expression.
So let's get rid of this dead code now that we fully support that
feature.

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