Registries are used to expand fields on elasticsearch
documents before they are presented to search API.
"document_collection" is a migrated format, so the field
"document_collections" should be expanded using content from
the govuk index, instead of the government index.

See PR where "document_collections" field was reinstated
for additional context:
#3215

The "policy_areas" field has been deprecated and is no longer
being indexed into elasticsearch [1]. There are no documents
in elasticsearch with this field populated in the govuk index,
so we no longer need the registry that expands the field.

[1] alphagov/whitehall#5666

Registries are used (in part) to expand fields on elasticsearch
documents before they are presented to search API. This works by
mapping the slug stored in the field to documents of the relevant
format, and taking additional information from the mapped document.

The "world_location" format is no longer being used: there
are no documents with this format stored in elasticsearch.
However, the field "world_locations" is used in many places
(for example as a filter on many finders) and the registry field
expansion changes the format of the field [1][2] from the raw
elasticsearch document from something that looks like:

"world_locations" : ["bulgaria"],

to something that looks like:

"world_locations": [{"slug": "bulgaria"}],

We don't yet understand the consequences of changing the formatting
on this field, so want to preserve it. As a quick way to keep this
formatting change, but remove references to the registry_index (as we
prepare to deprecate the government index), we can use the govuk
index to 'expand' the field. Even though it adds no additional
information (as before), this will preserve the formatting change.

[1] https://github.com/alphagov/search-api/blob/main/lib/search/presenters/entity_expander.rb#L75
[2] https://github.com/alphagov/search-api/blob/main/lib/search/presenters/field_presenter.rb#L33

Now all registries pull information from the govuk
index, we can remove the registry_index that points
to the government index.