remove unnecessary imports (#2456)

tutorials/image/cifar10/cifar10_eval.py

@@ -34,7 +34,6 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
-import argparse
 from datetime import datetime
 import math
 import time

tutorials/image/cifar10/cifar10_multi_gpu_train.py

@@ -39,7 +39,6 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
-import argparse
 from datetime import datetime
 import os.path
 import re

tutorials/image/cifar10/cifar10_train.py

@@ -36,7 +36,6 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
-import argparse
 from datetime import datetime
 import time