Clean up display_password_requirements feature flag

What does this MR do and why?

Feature has been deemed stable and the feature flag can be removed.

References

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MR acceptance checklist

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Before	After

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK.
• Make sure you have License locally (can be faked here by returning true).
• Sign out. Visit /users/sign_up.
• Fill the form with valid data.
• Fill the password with 111111112.
• Submit the form. Make sure user is created.

Related to #509916 (closed)