Cleanup: remove unused constraints attribute in Sequential model.

Author: fchollet

keras/models.py

@@ -662,12 +662,6 @@ def regularizers(self):
             self.build()
         return self.model.regularizers
 
-    @property
-    def constraints(self):
-        if not self.built:
-            self.build()
-        return self.model.constraints
-
     def get_weights(self):
         """Retrieves the weights of the model.