Remove cascading_auto_duo_code_review_settings feature flag

What does this MR do and why?

• This MR removes the cascading_auto_duo_code_review_settings feature flag
• Timeline:
  • cascading_auto_duo_code_review_settings was enabled on gitlab.com in %18.4
  • Pending BBM was finalized in %18.6 (%18.5 was the required stop): #561319 (closed)
  • So now we can safely remove the feature flag and have this setting available for self-managed and dedicated customers as well

How to set up and validate locally

• All specs should pass. We are just removing the feature flag.

MR acceptance checklist

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Related to #561318