remove internal Metric import as it is already in public API

Author: AakashKumarNain

tensorflow_addons/utils/keras_utils.py

@@ -21,7 +21,6 @@
 
 # TODO: find public API alternative to these
 from tensorflow.python.keras.losses import LossFunctionWrapper  # pylint: disable=unused-import
-from tensorflow.python.keras.metrics import Metric  # pylint: disable=unused-import
 
 
 def register_keras_custom_object(cls):