[588] part 2, delete One Login pre release banners flag from features

[elceebee]

Context

This PR removes an unused Feature flag from the code base. See earlier PR for full context.

Changes proposed in this pull request

No UI changes, just removing a feature from the database.

Guidance to review

Link to Trello card

https://trello.com/c/m4unLeJX

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