Bug 1622676 - Made dom/base flake8 compliant.r=championshuttler,sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D108310

.flake8

@@ -3,7 +3,6 @@ max-line-length = 99
 exclude =
     # These paths should be triaged and either fixed or moved to the list below.
     devtools/shared,
-    dom/base,
     dom/bindings,
     gfx/tests,
     ipc/ipdl/ipdl,

dom/base/gen-usecounters.py

@@ -6,7 +6,6 @@
 
 from __future__ import print_function
 
-import json
 import os
 import sys
 

dom/base/usecounters.py

@@ -2,11 +2,8 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-import buildconfig
 import collections
 import re
-from six import StringIO
-import sys
 
 
 def read_conf(conf_filename):