MAINT Remove unused condition (scikit-learn#8778)

Author: qinhanmin2014

sklearn/tree/tree.py

@@ -369,10 +369,6 @@ def fit(self, X, y, sample_weight=None, check_input=True,
 
     def _validate_X_predict(self, X, check_input):
         """Validate X whenever one tries to predict, apply, predict_proba"""
-        if self.tree_ is None:
-            raise NotFittedError("Estimator not fitted, "
-                                 "call `fit` before exploiting the model.")
-
         if check_input:
             X = check_array(X, dtype=DTYPE, accept_sparse="csr")
             if issparse(X) and (X.indices.dtype != np.intc or