docs: Remove unnecessary "omit in toc" labels

The label `<!-- omit in toc -->` was previously added to omit certain
headings from tables of contents generated by the Markdown All in One
extension for Visual Studio code. Since we started using MkDocs to build
the docs website, we no longer have tables of contents in raw markdown
files, thus the label is pointless.

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