MNT Remove useless operation in feature_importance_ (scikit-learn#13155)

Author: mgaido91

sklearn/ensemble/gradient_boosting.py

@@ -1634,8 +1634,7 @@ def feature_importances_(self):
                 normalize=False) for tree in stage) / len(stage)
             total_sum += stage_sum
 
-        importances = total_sum / len(self.estimators_)
-        importances /= importances.sum()
+        importances = total_sum / total_sum.sum()
         return importances
 
     def _validate_y(self, y, sample_weight):