Experiment Cleanup: Remove the `require_verification_for_namespace_creation` experiment

Summary

As per slack conversation we have decided to clean up this old Growth experiment.

From @alexbuijs:

This is a left-over from the Growth team. There are no plans to re-enable this experiment again. We re-used parts of it (credit card verification) for Identity Verification, so this feature can be removed.

• Experiment results: No results as this experiment was only briefly enabled and then rolled back to 0% (https://gitlab.com/gitlab-org/gitlab/-/issues/350251#note_1077142214)
  • Confirmed on the Experiment Event Validation this experiment was enabled 2022-03-30 and disabled 2022-04-07.
• Experiment key / feature flag name: require_verification_for_namespace_creation
• Epic or issue link:
  • gitlab-org/growth/team-tasks#554 (closed)
  • https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/533

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

We want to remove this experiment completely. Please tag @alexbuijs so he can review the final MR when ready!

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