chore: remove merge request reader FF check

What does this merge request do and why?

Removes the feature flag check for merge request reader as it has been fully rolled out, and we want to release it in 17.5.

Please see this conversation for context about why we want to remove it before the monolith changes.

How to set up and validate locally

Because this feature involves sending feature flags from the monolith, some of the testing can be done from there.

1. Enable feature flags on the monolith:
   • v2_chat_agent_integration
   • ai_merge_request_reader_for_chat

Feature.enable(:v2_chat_agent_integration)
Feature.enable(:ai_merge_request_reader_for_chat)

2. Test MR feature flag enabled:

   • Go to a merge request
   • Ask: "What files were changed on this MR?"
   • Expected result: It should reply with the list of changed files

3. Run /clear to reset the chat

4. Disable MR feature flag on the monolith:

   • Turn off ai_merge_request_reader_for_chat on the monolith

Feature.disable(:ai_merge_request_reader_for_chat)

5. Test MR feature flag disabled:
   • Go to a merge request
   • Ask: "What files were changed on this MR?"
   • Expected result: You will get an error, because the feature has not been removed in the monolith yet. However, if you look at the logs, you will see that it tried to use the merge request reader tool. This scenario is not a problem because it is globally enabled on .com, and we are planning to fully clean up the FF in 17.5.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.