Remove last of the missing file placeholders

During conversion, when we hit a dangling reference to a page that didn't
exist, a placeholder was added for that page to allow Hugo to generate the
site. We now want to remove these pages.

 - Test cases: Replaced with "obsolete test" page.
 - Old bug links: Converted to broken link.
 - Link to specs removed.
 - Links to yet-to-be-created release notes removed.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>