Drop the ToC from docs/data-guidelines.md (#9896)
Drop the ToC from docs/data-guidelines.md (#9896)

Per https://github.blog/changelog/2021-04-13-table-of-contents-support-in-markdown-files/
the GitHub UI now provides a button for displaying a generated ToC for
all markdown files — making our in-document ToC in the docs/data-guidelines.md
file somewhat redundant. So this change removes it.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data