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>

Documentation/scheduler/completion.txt

@@ -25,8 +25,7 @@ struct completion that tells the waiting threads of execution if they
 can continue safely.
 
 As completions are scheduling related, the code is found in
-kernel/sched/completion.c - for details on completion design and
-implementation see completions-design.txt
+kernel/sched/completion.c.
 
 
 Usage: