Remove unused files.

demo/bert/train_student.py

-import paddle.fluid as fluid
-from paddleslim.nas.darts.search_space import ConvBERTClassifier
-
-place = fluid.CUDAPlace(fluid.dygraph.parallel.Env().dev_id)
-
-with fluid.dygraph.guard(place):
-
-    bert = ConvBERTClassifier(3)
-    bert.fit("./data/glue_data/MNLI/",
-             5,
-             batch_size=16,
-             use_data_parallel=False,
-             learning_rate=0.00005,
-             save_steps=1000)