atomic: remove further references to atomic_ops

Commit f0400a77 ("atomic: Delete obsolete documentation") removed
./Documentation/core-api/atomic_ops.rst, but missed to remove further
references to that file.

Hence, make htmldocs warns:

  Documentation/core-api/index.rst:53: WARNING:
  toctree contains reference to nonexisting document 'core-api/atomic_ops'

Also, ./scripts/get_maintainer.pl --self-test=patterns warns:

  warning: no file matches    F:    Documentation/core-api/atomic_ops.rst

Remove further references to ./Documentation/core-api/atomic_ops.rst.

Fixes: f0400a77

 ("atomic: Delete obsolete documentation")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20201220060927.21582-1-lukas.bulwahn@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>