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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/scheduler/completion.txt

···
 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: