Remove serving_dirs usage in speedometer2

The serving dir is also included by the page.

Bug: chromium:883798
Change-Id: I1d1a512774b69312642b6226a80023ac88ff3293
Reviewed-on: https://chromium-review.googlesource.com/1226752Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Vovo Yang <vovoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591301}

tools/perf/benchmarks/speedometer2.py

@@ -155,8 +155,7 @@ class Speedometer2(perf_benchmark.PerfBenchmark):
                                      self.enable_smoke_test_mode)
 
   def CreateStorySet(self, options):
-    ps = story.StorySet(base_dir=_SPEEDOMETER_DIR,
-        serving_dirs=[_SPEEDOMETER_DIR])
+    ps = story.StorySet(base_dir=_SPEEDOMETER_DIR)
     ps.AddStory(page_module.Page(
        'file://InteractiveRunner.html', ps, ps.base_dir, name='Speedometer2'))
     return ps