mm/damon/Kconfig: Hide PAGE_IDLE_FLAG from users

mm/damon/Kconfig: Hide PAGE_IDLE_FLAG from users

Commit 2a058a1a9914 ("mm/idle_page_tracking: make PG_idle reusable") of
linux-mm[1] makes CONFIG_PAGE_IDLE_FLAG option to be presented to the
user.  However, the option is hard to be understood by users.  Also, it is
not intended to be set by users but other kernel subsystems like DAMON or
IDLE_PAGE_TRACKING.  To avoid confusions, this commit removes the prompt
so that the option is not presented to the user.

[1] https://github.com/hnaz/linux-mm/commit/2a058a1a9914

Link: https://lkml.kernel.org/r/20210813081238.34705-1-sj38.park@gmail.com
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>