Removes update_statistics_namespace feature flag

What does this MR do?

Removes update_statistics_namespace feature flag

After measuring the impact of the namespace storage statistics on https://gitlab.com/gitlab-org/gitlab-ce/issues/64092. It was decided that it's performant enough. So we can freely remove the feature flag

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/64092

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers