[545] Remove feature flag uses throughout

[elceebee]

Context

trello: https://trello.com/c/0ZWcB3K9

This is the first part of a clean up task to remove the feature flag and old withdraw routes and views once the feature has been fully released.

The second PR, here, will remove the feature flag from the database.

Neither PR should be merged until the feature flag has been activated (plan to activate on 13 Jan, so this can be merged in a few days after that if all is going well)

Changes proposed in this pull request

• Removes feature flag uses
• Updates tests
• Removes views / routes that are no longer in use

Guidance to review

Any missing test coverage / unintended consequences?

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated.
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card