Remove specialist topic code from specs

[unoduetre]

What

This prepares for https://github.com/alphagov/email-alert-api/pull/2136 and is related to https://github.com/alphagov/gds-api-adapters/pull/1250

Why

This PR has been extracted from https://github.com/alphagov/email-alert-api/pull/2136 as requested by @hannako

Trello ticket

[unoduetre]

@hannako

I've checked and ContentChange also uses taxon_tree. This is added by this code in email-alert-service. This simplified things a bit. If a test already uses taxon_tree, I've used document_collections instead, as those are also used by the database entries.