[scheduler] Remove redundant callback clear in delete_item_

The destructor of std::function already handles releasing captured
resources — no need to explicitly null it before delete.

esphome/core/scheduler.h

@@ -353,13 +353,8 @@ class Scheduler {
   // IMPORTANT: Caller must hold the scheduler lock before calling this function.
   void recycle_item_main_loop_(SchedulerItem *item);
 
-  // Helper to delete a SchedulerItem (clears callback then frees memory)
-  static void delete_item_(SchedulerItem *item) {
-    if (item != nullptr) {
-      item->callback = nullptr;
-      delete item;
-    }
-  }
+  // Helper to delete a SchedulerItem
+  static void delete_item_(SchedulerItem *item) { delete item; }
 
   // Helper to perform full cleanup when too many items are cancelled
   void full_cleanup_removed_items_();