Remove `use_holt_winters_forecast_for_deployment_frequency` feature flag

What does this MR do and why?

This feature was created for an experiment that we decided not to move forward with. This MR removes the feature flag and all related code. This only removes the Holt Winters forecasting experiment, the original linear regression forecasting will still be intact.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

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

Screenshots or screen recordings

Forecast off	Forecast on

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Navigate to the Analyze > CI/CD Analytics page for any project or group
• Click the Deployment Frequency tab
• Toggle the Show forecast button and ensure the chart updates accordingly

Related to #507427 (closed)