Remove dev feature flag from GET /users API

Issue Description

This API has been live on prod for a long time but the feature flag has not been removed.
Remove the dev FF from backend and all other repos where its being used.

Reason for adding dev FF

1. To get user by discord id through a query param for /user discord command
2. Change query param with name query to q

Usages in other repos

1. discord-slash-commands - Partial use 🟨
2. members-site - None of the usages are using dev FF 🟥
3. mobile-app - None of the usages are using dev FF 🟥
4. website-dashboard - None of the usages are using dev FF 🟥
5. website-status - None of the usages are using dev FF 🟥

This is a subtask of #2075

Expected Behavior

Current Behavior

Screenshots

NA

Reproducibility

• This issue is reproducible
• This issue is not reproducible

Steps to Reproduce

NA

Severity/Priority

• Critical
• High
• Medium
• Low

Additional Information

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.

Start by searching the website-backend repository for the dev feature flag and the GET /users endpoint, then check its usages in discord-slash-commands, members-site, mobile-app, website-dashboard, and website-status. Confirm which usages still depend on the flag and the query parameters before removing it from the backend and affected repositories. Done means the flag is removed everywhere it is used without losing the Discord ID lookup or the q query parameter behavior.