Experiment Cleanup: Remove the `runners_availability_section` experiment

Summary

As per conversation in the rollout issue we have decided to clean up this experiment.

• Experiment results: This experiment has been running at 50% since March 17, 2022. We did not do an in-depth analysis of this experiment in service of developing a standard pattern for empty states & cleaning up old experiments. This will be tackled here: #396716
• Experiment key / feature flag name: runners_availability_section
• Epic or issue link:
  • gitlab-org/growth/team-tasks#465 (closed)
  • #355127 (closed)

Changes to implement as part of the roll out to becoming an official part of the product:

We want to remove this experiment completely.

Steps

• Determine whether the feature should apply to SaaS and/or self-managed
  • n/a as we are removing
• Determine whether the feature should apply to EE - and which tiers - and/or Core
  • n/a as we are removing
• Determine if tracking should be kept as is, removed, or modified.
  • Removed
• Ensure any relevant documentation has been updated.
  • No documentation updates needed
• Consider changes to any feature_category: introduced by the experiment if ownership is changing (PM for Growth and PM for the new category as DRIs)
  • n/a as we are removing
• Check to see if the experiment introduced new design assets. Add them to the appropriate repos and document them if needed.
  • n/a as we are removing
• Optional: Migrate experiment to a default enabled feature flag for one milestone and add a changelog. Converting to a feature flag can be skipped at the ICs discretion if risk is deemed low with consideration to both SaaS and (if applicable) self managed
  • n/a as we are removing
• In the next milestone, remove the feature flag if applicable
• After the flag removal is deployed, clean up the feature/experiment feature flags by running chatops command in #production channel
• Ensure the corresponding Experiment Rollout issue is updated
• [ ]