atomic: remove further references to atomic_ops

Commit f0400a77eb ("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: f0400a77eb ("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>

Documentation/core-api/index.rst

@@ -53,7 +53,6 @@ How Linux keeps everything from happening at the same time.  See
 .. toctree::
    :maxdepth: 1
 
-   atomic_ops
    refcount-vs-atomic
    irq/index
    local_ops

MAINTAINERS

@@ -10260,7 +10260,6 @@ S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
 F:	Documentation/atomic_bitops.txt
 F:	Documentation/atomic_t.txt
-F:	Documentation/core-api/atomic_ops.rst
 F:	Documentation/core-api/refcount-vs-atomic.rst
 F:	Documentation/litmus-tests/
 F:	Documentation/memory-barriers.txt