[gerrit_util.py] Remove unnecessary imports from gerrit_util.py

Remove the libraries that have no uses according to gd in vim.

BUG=None
TEST=upload this CL

Change-Id: I7fe6bbd54c1a4f00a3d0e3f04be0f02609734e46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5578897
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gregory Nisbet <gregorynisbet@google.com>

gerrit_util.py

@@ -17,8 +17,6 @@ import os
 import random
 import re
 import socket
-import stat
-import sys
 import tempfile
 import time
 import urllib.parse
@@ -29,7 +27,6 @@ import gclient_utils
 import metrics
 import metrics_utils
 import scm
-import subprocess2
 
 import http.cookiejar
 from io import StringIO