Remove disable_ropc_for_new_applications feature flag

What does this MR do and why?

• Remove the disable_ropc_for_new_applications feature flag
• Result:
  • In any EE environment where the feature is available, new Doorkeeper::Applications get ropc_enabled=false
  • In other environments, new Doorkeeper::Applications continue to get ropc_enabled=true.

This aligns with the intended functionality, where ROPC is disabled for all new OAuth apps on GitLab.com.

References

• FF issue: #545459 (closed)
• Original MR: !192410 (merged)
• Follow-up MR to remove disable_ropc_for_all_applications: !205939 (merged)

How to set up and validate locally

1. Simulate SaaS
2. Create a new OAuth application Admin area --> Applications
3. In a Rails console, observe the new application got ropc=false

gdk rails c
> Doorkeeper::Application.last
...

4. 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."}

This confirms the functionality to disable ROPC for new apps works without the new feature flag.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #545459 (closed)