docs/vm/damon: remove broken reference

mainline inclusion
from mainline-5.16
commit 876d0aac
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4GVMK
CVE: NA

-------------------------------------------------
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>
(cherry picked from commit 876d0aac)
Signed-off-by: Yue Zou <zouyue3@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>