[df] Avoid clearing sample callbacks after event loop

Sample callbacks can be registered by an RAction or an RDefinePerSample
instance. In both cases, the lifetime of the callback is tied to the lifetime of
the object itself. Avoid eager clearing of the callbacks so to not interfer with
the normal functioning.

tree/dataframe/src/RLoopManager.cxx

@@ -761,7 +761,6 @@ void RLoopManager::CleanUpNodes()
 
    fCallbacks.clear();
    fCallbacksOnce.clear();
-   fSampleCallbacks.clear();
 }
 
 /// Perform clean-up operations. To be called at the end of each task execution.