Remove disable_ropc_for_all_applications feature flag

What does this MR do and why?

• Remove the disable_ropc_for_all_applications feature flag
• The flag has been at 100% in gprd since Sep. 12 with no issues #566300 (closed) https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/48739
• Result:
  • In any EE environment where the feature is available, ROPC requests are globally disabled
  • In other environments, ROPC is not globally disabled

This aligns with the intended functionality: OAuth ROPC flow is disabled globally on GitLab.com.

References

• FF issue: #566300 (closed)
• Original MR: !203145 (merged)

How to set up and validate locally

1. Simulate SaaS
2. Create an OAuth application via Admin area --> Applications
3. Test OAuth password grant flow, observe you get HTTP 401 response:

echo "grant_type=password&username=<your_username>&password=<your_password>" > auth.txt
curl --data "@auth.txt" --user <oauth_application_id>:<oauth_application_secret> --request POST "https://<your_gdk_url>/oauth/token"
{"error":"unauthorized_client","error_description":"The client is not authorized to perform this request using this method."}

MR acceptance checklist

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Related to #566300 (closed)