remove all references to weightedDiffMeasure from cluster_telemetry code

cluster_telemetry/telemetry_slave_scripts/mocks/imagediffdb.py

 #!/usr/bin/env python
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Dummy module that pretends to be imagediffdb.py for write_json_summary_test.
 
 imagediffdb.py here refers to
 https://code.google.com/p/skia/source/browse/trunk/gm/rebaseline_server/imagediffdb.py
 
@@ skipping 10 matching lines @@
                actual_image_locator=None, expected_images_subdir=None,
                actual_images_subdir=None):
     pass
 
   def get_num_pixels_differing(self):
     return 1
 
   def get_percent_pixels_differing(self):
     return 2
 
-  def get_weighted_diff_measure(self):
-    return 3
-
   def get_max_diff_per_channel(self):
     return 4
 
   def get_perceptual_difference(self):
     return 5
 
 
 class ImageDiffDB(object):
 
   def __init__(self, storage_root):
     pass
 
   def add_image_pair(self, expected_image_url, expected_image_locator,
                      actual_image_url, actual_image_locator):
     pass
 
   def get_diff_record(self, expected_image_locator, actual_image_locator):
     return DiffRecord()