netfilter: nf_tables: add helper to unregister chain hooks

This patch adds a helper function to unregister chain hooks in the chain
deletion path. Basically, a code factorization.

The new function is useful in follow-up patches.

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