docs: Remove table of contents

Some of the section links in `docs/Prerequisites.md`'s table of contents
are broken. Other files in `docs` do not contain tables of contents.

Rather fixing and adding tables, we remove the existing one to reduce
maintenance effort. Many Markdown viewers (such as the repository
viewers of GitLab and GitHub) and documentation webpage generators
(e.g. MkDocs) automatically show each page's table of content.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>