mm/damon/sysfs: remove damon_sysfs_cmd_request and its readers

mm/damon/sysfs: remove damon_sysfs_cmd_request and its readers

damon_sysfs_cmd_request is DAMON sysfs interface's own synchronization
mechanism for accessing DAMON internal data via damon_callback hooks.  All
the users are now migrated to damon_call() and damos_walk(), so nobody
really uses it.  No one writes to the data structure but reading code is
still remained.  Remove the reading code and the entire data structure.

Link: https://lkml.kernel.org/r/20250306175908.66300-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>