rebaseline_server: use just skpdiff, not Python Image Library

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

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@@ skipping 92 matching lines @@
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     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
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         "numDifferingPixels": 120000, 
         "percentDifferingPixels": 75.0, 
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       "imageBUrl": "bitmap-64bitMD5/filterbitmap_checkerboard_32_2/15528304435129737588.png", 
       "isDifferent": true
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@@ skipping 388 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"
     }
   }
 }