Remove refactor_vulnerability_filters feature flag

What does this MR do and why?

In 15.10 !112756 (merged) enabled strict enforcement of IP allow list on who can create pipelines. This also extends to other areas of IP policy enforcement such as ability to create projects and will help prevent any vulnerabilities within that enforcement. One such issue was reported and therefore it's suggested to backport this FF enablement to 15.10 (also see discussion for why this isn't targeted at a security release)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Ensure the e2e:package-and-test job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

• Refer to the internal pilot issue for feedback or questions.
• Refer to the patch release runbook for engineers and maintainers for guidance.