Fix or remove Background Textures

[gestchild]

What

We have a document type in Prismic called background-textures, that allows us to upload images which can then be selected as background in page headers.

It looks like these are intended to be used for events and event-series. Both these custom types have a content relationship field for pulling in one of these background-textures.

However, these don't display for either of these pages and it doesn't look like they have for some time.

• the event-series fetcher doesn't include the the necessary fetchlinks to return the data (although it does look for it in the transformer)
• the event fetcher fetches the necessary data, but doesn't do anything with it in the transformer.

As part of the work for #10781, I've commented out the transformer code which was causing typing errors, rather than trying to fix this as part of that work.

We need to decide if we need this functionality and, if not,

• disable the background-textures type in Prismic,
• remove the content relationship fields from events and event-series
• remove the fetchLinks and related types from the codebase
• remove the commented out code for transforming background-textures data

If we want to have this functionality, we need to fix it:

• fetch the data for the event-series
• transform the data for events
• make sure we're rendering it on the page