Remove manually managed ToC in blueprint

What does this MR do and why?

What

Remove the manually generated table of content.

Why

The documentation page https://docs.gitlab.com/ee/architecture/blueprints/cells/infrastructure/cell_arch_tooling.html generates the ToC on it's own on the side.

Also, GitLab Markdown can generate the Table of Content [automatically with \[TOC\] if we want.

Managing it manually can get outdated pretty fast and requires the writer to remember when they update the heading or add a new one.