feat(feature-manager): tear out AggregateStats feature

[mantariksh]

Problem

As part of removing the feature manager, the AggregateStats feature needs to be removed. This feature controls whether the Examples page relies on the Submissions collection or the FormStatisticsTotal collection for its queries.

Part of #1842

Solution

The approach was to:

1. Tear out the Examples service's reliance on the RetrievalType enum, which was in turn determined by the AGGREGATE_COLLECTION env var. Since there is now only one RetrievalType, the higher-order functions which used to take in a RetrievalType and return a service function were converted into regular functions.
2. Tear out the Examples factory and link the controllers directly to the service.
3. Remove mentions of AGGREGATE_COLLECTION throughout the documentation and dev environment.

For reviewers

If you wish to review the test files in detail, the changes may make more sense by commit. cc207db does the actual work of updating the tests, whereas e821d0b just removes certain describe blocks which are no longer relevant due to mentions of the AggregateStats feature.

Breaking Changes

• Yes - this PR contains breaking changes
  • Details ...
• No - this PR is backwards compatible
  • Server-side only. Note that this PR can be merged independently of feat(feature-manager): remove feature toggles from frontend #2118 in a backwards-compatible manner, since the frontend does not rely on this env var.

Deprecated environment variables:

• AGGREGATE_COLLECTION: previously used to toggle between using the Submissions and FormStatisticsTotal collections for calculating examples page statistics

[karrui]

lgtm