Clean up dora_charts_forecast feature flag

What does this MR do and why?

Removes the dora_charts_forecast feature flag – it's been default enabled for a while now, and it was determined that the functionality will remain in production. It also removes the Experiment badge that was located next to to the Show forecast toggle.

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.

• #507528 (closed)

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

Before	After

How to set up and validate locally

1. Please make sure that you're on the GitLab Ultimate plan.
2. Seed data: FILTER=ci_cd_analytics SEED_CI_CD_ANALYTICS=1 bundle exec rake db:seed_fu
3. Open the group/project URL in the output of the command above
4. Navigate to Analyze > CI/CD analytics, then select the Deployment Frequency tab
5. Verify that you're able to toggle the Show forecast switch and enable data forecasting as shown in the chart below.

Related to #507528 (closed)