Remove current_page_context_prompt_in_aigw feature flag

What does this MR do and why?

Remove current_page_context_prompt_in_aigw feature flag

Changelog: other EE: true

References

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• [Feature flag] Enable current_page_context_prom... (#517316 - closed) • Tetiana Chupryna • 17.9
• Move AI context prompts to AI Gateway (#508317 - closed) • Tetiana Chupryna • 17.9

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Related to #517316 (closed)