Remove unused group MR analytics code

What does this MR do and why?

We currently have a JavaScript entrypoint, HAML template and sidebar menu item for Group-Level "Merge request analytics". These are unused however, given that there isn't even a controller. If we ever want to introduce this kind of analytics, we can just recreate the files.

Screenshots or screen recordings

N/A

How to set up and validate locally

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MR acceptance checklist

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