docs/completion.txt: drop dangling reference to completions-design.txt

Per the original author, the proposed document was never deemed
necessary, and the important bits got merged into completion.txt. Let's
just stop confusing readers by pointing at a nonexistent doc.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>