rebaseline_server: delete no-longer-used weightedDiffMeasure

gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json

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           13
@@ skipping 92 matching lines @@
           2
         ], 
         [
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   "header": {
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     "schemaVersion": 3, 
     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
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         "percentDifferingPixels": 75.0, 
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-        "weightedDiffMeasure": 8.413046264257337
+        "perceptualDifference": 50.122499999999995
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       "expectations": {
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-        "weightedDiffMeasure": 14.005823726182287
+        "perceptualDifference": 25.120500000000007
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       }, 
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         "percentDifferingPixels": 12.035778985507246, 
-        "perceptualDifference": 100, 
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+        "perceptualDifference": 100
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       }, 
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       }, 
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       "isDifferent": true
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         "percentDifferingPixels": 12.17685688405797, 
-        "perceptualDifference": 100, 
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@@ skipping 389 matching lines @@
     "imageB": {
       "baseUrl": "http://chromium-skia-gm.commondatastorage.googleapis.com/gm", 
       "description": "actual image"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }