Remove serving_dirs usage in static_top_25_pages

The serving dir is also included by the pages.

Bug: chromium:883798
Change-Id: Ifb67e19b03303e4929a0e588f5450707adec4835
Reviewed-on: https://chromium-review.googlesource.com/1226484Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Vovo Yang <vovoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591312}

tools/perf/page_sets/static_top_25_pages.py

@@ -14,7 +14,6 @@ class StaticTop25PageSet(story.StorySet):
 
   def __init__(self):
     super(StaticTop25PageSet, self).__init__(
-        serving_dirs=[STATIC_TOP_25_DIR],
         cloud_storage_bucket=story.PARTNER_BUCKET)
 
     shared_desktop_state = shared_page_state.SharedDesktopPageState