!2731 remove export lite model api

Merge pull request !2731 from yangjie159/r0.5

mindspore/train/serialization.py

@@ -420,7 +420,6 @@ def export(net, *inputs, file_name, file_format='GEIR'):
             - GEIR: Graph Engine Intermidiate Representation. An intermidiate representation format of
               Ascend model.
             - ONNX: Open Neural Network eXchange. An open format built to represent machine learning models.
-            - LITE: Huawei model format for mobile. A lite model only for the MindSpore Lite
             - BINARY: Binary format for model. An intermidiate representation format for models.
     """
     logger.info("exporting model file:%s format:%s.", file_name, file_format)