fix(__future__): Decommission the experimental update-flag

We shipped an experimental `update-flag` endpoint (issue) and later decided to stabilise options A and B paths onto a new `/api/__future__/environment/{key}/features/{id}/` (C) option (issue).

The code for the previous implementation still remains.

## Acceptance criteria

- Code currently depending on any components of the old update-flag is adjusted to either break the dependency, or reroute it to option C.
- All the code that power the old update-flag, i.e. both options A and B, is deleted.

Start by tracing the old update-flag endpoint and the components serving options A and B, then identify code that still depends on them. Reroute dependencies to `/api/__future__/environment/{key}/features/{id}/` where appropriate, delete the old implementation, and verify that no update-flag code remains.

Written by the indexing model from the issue text.