Remove symlinked directories and excess files in /sites/blog

Why is this change being made?

This change removes all symlinks under site/blog that were added to create a separate middle instance for just the blog.

This MR also removes the following files:

• Removes sites/blog/config.rb
• Removes extensions/partial_build_blog.rb because it's no longer needed

These files are being removed as part of the monorepo refactor preliminary separation and decoupling tasks. It will also prepare the repository for phase 1 of the www-gitlab-com monorepo refactor epic.

Removing these files also disable the possibility of running just the blog while developing locally.

Author Checklist

• Correct MR template applied (e.g blog post)
• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

For help with failing pipelines reach out in #mr-buddies in Slack

Related #8302 (closed)