remove some not used imports from python codes

hack/boilerplate/boilerplate.py

@@ -19,8 +19,6 @@ from __future__ import print_function
 import argparse
 import difflib
 import glob
-import json
-import mmap
 import os
 import re
 import sys

hack/update_owners.py

@@ -17,10 +17,10 @@
 import argparse
 import collections
 import csv
-import re
 import json
 import os
 import random
+import re
 import subprocess
 import sys
 import time

hack/verify-flags-underscore.py

@@ -16,12 +16,10 @@
 
 from __future__ import print_function
 
-import json
-import mmap
+import argparse
 import os
 import re
 import sys
-import argparse
 
 parser = argparse.ArgumentParser()
 parser.add_argument("filenames", help="list of files to check, all files if unspecified", nargs='*')