Remove Pages redirect for 15.1x

What does this MR do and why?

We're seeing some odd redirect behavior on URLs that end in 15.1x:

• https://docs.gitlab.com/15.11 -> https://archives.docs.gitlab.com/15.1/1
• https://docs.gitlab.com/15.10 -> https://archives.docs.gitlab.com/15.1/0
• These should go to https://archives.docs.gitlab.com/15.11 and https://archives.docs.gitlab.com/15.10, respectively.

We provide redirects for archived pages via the frontend 404 handler here, but this redirect is actually coming from the webserver/GitLab Pages:

$ curl -I https://docs.gitlab.com/15.11
HTTP/2 301
content-type: text/html; charset=utf-8
location: https://archives.docs.gitlab.com/15.1/1
permissions-policy: interest-cohort=()
vary: Origin
date: Tue, 27 Feb 2024 22:43:28 GMT

It must be something up with the splat rule that this MR removes. This might be a Pages bug? In any case, we can remove the Pages redirect and have our frontend handler take this over.

Closes #1771 (closed)

How to set up and validate locally

Can't test this locally. We'll verify it once it deploys. Hopefully the broken redirects are not cached for too long. 🤞 (Edit: I think 301s like this are cached indefinitely 😞 people will have to clear their browser cache to see the right behavior)

Merge request 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 merge request.

Closes #1771 (closed)