[Experiment Cleanup] invite_members_in_side_nav

Summary

As per PM conversation we have decided to clean up this experiment.

• Experiment results: We are unable to analyze results of this experiment due to the age and it appears the experiment was never fully rolled out https://gitlab.com/gitlab-org/gitlab/-/issues/341309
• Experiment key / feature flag name: invite_members_in_side_nav
• Epic or issue link: &3718 (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