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

atomic_ops.rst was removed by commit f0400a77ebdc ("atomic: Delete
obsolete documentation").
Remove the broken link in tools/memory-model/Documentation/simple.txt.

Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

tools/memory-model/Documentation/simple.txt

···
 
 Documentation/atomic_t.txt
 Documentation/atomic_bitops.txt
-Documentation/core-api/atomic_ops.rst
 Documentation/core-api/refcount-vs-atomic.rst
 
 Reading code using these primitives is often also quite helpful.