tools/memory-model: Remove reference to atomic_ops.rst

atomic_ops.rst was removed by commit f0400a77

 ("atomic: Delete
obsolete documentation").
Remove the broken link in tools/memory-model/Documentation/simple.txt.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>