Do not clear entities callbacks on destruction (ros2#2002)

* Do not clear entities callbacks on destruction

Removing these clearings since they were not necessary,
since the objects are being destroyed anyway.

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>

* Fix CI

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>

* Restore clear_on_ready_callback on ~QOSEventHandlerBase

Needed since QOSEventHandlerBase does not own
the pub/sub listeners. So the QOSEventHandler
can be destroyed while the corresponding listeners
are still alive, so we need to clear these callbacks.

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>

* Add coment on clearing callback for QoS event

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>
Co-authored-by: Mauro Passerino <mpasserino@irobot.com>

rclcpp/src/rclcpp/event_handler.cpp

@@ -39,6 +39,15 @@ UnsupportedEventTypeException::UnsupportedEventTypeException(
 
 EventHandlerBase::~EventHandlerBase()
 {
+  // Since the rmw event listener holds a reference to
+  // this callback, we need to clear it on destruction of this class.
+  // This clearing is not needed for other rclcpp entities like pub/subs, since
+  // they do own the underlying rmw entities, which are destroyed
+  // on their rclcpp destructors, thus no risk of dangling pointers.
+  if (on_new_event_callback_) {
+    clear_on_ready_callback();
+  }
+
   if (rcl_event_fini(&event_handle_) != RCL_RET_OK) {
     RCUTILS_LOG_ERROR_NAMED(
       "rclcpp",