mm/damon/sysfs: remove damon_sysfs_cmd_request_callback() and its callers

mm/damon/sysfs: remove damon_sysfs_cmd_request_callback() and its callers

damon_sysfs_cmd_request_callback() is the damon_callback hook functions
that were used to handle user requests that need to read and/or write
DAMON internal data.  All the usages are now updated to use damon_call()
or damos_walk(), though.  Remove it and its callers.

Link: https://lkml.kernel.org/r/20250306175908.66300-7-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>