docs/vm/damon: remove broken reference

Building DAMON documents warns for a reference to nonexisting doc, as
below:

    $ time make htmldocs
    [...]
    Documentation/vm/damon/index.rst:24: WARNING: toctree contains reference to nonexisting document 'vm/damon/plans'

This fixes the warning by removing the wrong reference.

Link: https://lkml.kernel.org/r/20210917123958.3819-4-sj@kernel.org


Signed-off-by: SeongJae Park <sjpark@amazon.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>