Remove topics, additional_topics and primary topic from email_alert schema

[unoduetre]

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Follow these steps if you are doing a Rails upgrade.

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

What

The tags field of the email alert schema contains the following keys: topics, additional topics and primary topic. This PR is about:

• Confirming all three of these are specialist topic fields. There’s this.
• Removing the fields following https://github.com/alphagov/publishing-api/blob/main/docs/content_schemas/changing-an-existing-content-schema.md#removing-a-field-from-a-content-schema

Why

Trello card

How

see steps above

[hannako]

Thanks Mat - looks good to me 🏅

[brucebolt]

Looks good to me. Would you be able to add an extra comment to the second commit message explaining what was done (e.g. auto-regenerating the schema files).

[unoduetre]

Looks good to me. Would you be able to add an extra comment to the second commit message explaining what was done (e.g. auto-regenerating the schema files).

I've added a longer explanation to that commit.