update_payload: Remove assertIsNone.

unittest.assertIsNone was introduced in python 2.7 but at the time
our servers were running an older python version. We have migrated to
python 2.7 so we can drop the local wrapper now.

BUG=None
TEST=ran unittest.

Change-Id: I1c43d027361ab7f8f5fad83cc900361e5a6ee137
Reviewed-on: https://chromium-review.googlesource.com/253610
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
diff --git a/scripts/update_payload/checker_unittest.py b/scripts/update_payload/checker_unittest.py
index c099e2a..60ae1a2 100755
--- a/scripts/update_payload/checker_unittest.py
+++ b/scripts/update_payload/checker_unittest.py
@@ -132,19 +132,6 @@
       new_field = repeated_field.add()
       new_field.CopyFrom(field_val)
 
-  # The production environment uses an older Python, this isn't an override.
-  # pylint: disable=W0221
-  def assertIsNone(self, val):
-    """Asserts that val is None (TODO remove once we upgrade to Python 2.7).
-
-    Note that we're using assertEqual so as for it to show us the actual
-    non-None value.
-
-    Args:
-      val: Value/object to be equated to None.
-    """
-    self.assertIs(None, val)
-
   def SetupAddElemTest(self, is_present, is_submsg, convert=str,
                        linebreak=False, indent=0):
     """Setup for testing of _CheckElem() and its derivatives.