Remove topic_taxonomy_taxons from expanded links

[unoduetre]

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Part of https://trello.com/c/2LcbNRSQ/2417-%F0%9F%8F%94epic-remove-specialist-topic-code-from-publishing-api-l

What

topic_taxonomy_taxons appear to be a legacy field that stored a mapping between specialist topics and taxonomy topics. So we should retire them.

Why

Remove tech debt
Trello ticket

How

See https://github.com/alphagov/publishing-api/blob/main/docs/content_schemas/changing-an-existing-content-schema.md

It has been verified by @hannako that there are no topic_taxonomy_taxon links in the database—please, see the comment below.

Link.where(link_type: "topic_taxonomy_taxons").count 
=> 0

The following search among all of our GitHub repositories shows that this field is no longer used:
org:alphagov topic_taxonomy_taxons
The returned search result from the whitehall repository is unrelated as it's just a name of a cache key:

    TAXONS_CACHE_KEY = "topic_taxonomy_taxons".freeze

[hannako]

This looks good, thanks Mat.

I checked in the publishing API database:

Link.where(link_type: "topic_taxonomy_taxons").count 
=> 0

And this also suggests there's nothing in the frontend rendering or email alerts that's relying on this field.

Please could you add the above into the PR description.

[jkempster34]

LGTM.

Thanks for splitting up the commits!