Consolidate AI-related feature flags (part 2)

What does this MR do and why?

This MR removes two feature flags related to experimental AI features. Related to #425690 (closed). I've also added some specs for a fuller coverage.

Removing the feature flags does NOT release the features as they are still all gated behind the "umbrella" feature flag openai_experimentation for all AI features. For this reason, the changelog value is other.

The removed feature flags are listed here:

1. summarize_comments: user can summarize the comments of an issue using a button.

   Rollout issue: #406758 (closed)

   Status: enabled globally but not enabled by default

2. generate_description_ai: user can write a short description and use the feature to fill in a template based on the description.

   Rollout issue: #409369 (closed)

   Status: enabled globally but not enabled by default

How to set up and validate locally

1. First verify that the features gated behind the removed feature flags are still working only with the FF openai_experimentation enabled.

FF :summarize_comments	FF :generate_description_ai (the tanuki icon)

2. Disable the FF openai_experimentation and check the buttons don't get rendered

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #425690 (closed)