mm/damon: remove damon_callback->private

The field was added to let users keep their personal data to use inside of
the callbacks.  However, no one is actively using that now.  Remove it.

Link: https://lkml.kernel.org/r/20250306175908.66300-10-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

include/linux/damon.h

@@ -609,12 +609,10 @@ struct damon_operations {
  * @after_aggregation:	Called after each aggregation.
  * @before_damos_apply:	Called before applying DAMOS action.
  * @before_terminate:	Called before terminating the monitoring.
- * @private:		User private data.
  *
  * The monitoring thread (&damon_ctx.kdamond) calls @before_start and
  * @before_terminate just before starting and finishing the monitoring,
- * respectively.  Therefore, those are good places for installing and cleaning
- * @private.
+ * respectively.
  *
  * The monitoring thread calls @after_wmarks_check after each DAMON-based
  * operation schemes' watermarks check.  If users need to make changes to the
@@ -630,8 +628,6 @@ struct damon_operations {
  * If any callback returns non-zero, monitoring stops.
  */
 struct damon_callback {
-	void *private;
-
 	int (*before_start)(struct damon_ctx *context);
 	int (*after_wmarks_check)(struct damon_ctx *context);
 	int (*after_sampling)(struct damon_ctx *context);