Remove unnecessary ordering attribute from FeedIsSynthetic

Bug: 758782
Change-Id: I4b7ff5cd8116d9b88ae1e9fe12adb7532a6ecf4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466840
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816512}

tools/metrics/histograms/histograms_xml/histogram_suffixes_list.xml

@@ -6320,7 +6320,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
       name="ContentSuggestions.Feed.VisualElement.Viewed.TimeSinceElementFetched"/>
 </histogram_suffixes>
 
-<histogram_suffixes name="FeedIsSynthetic" separator="." ordering="suffix">
+<histogram_suffixes name="FeedIsSynthetic" separator=".">
   <suffix name="NotSynthetic"
       label="Continuations that require making remote requests to fetch more
              articles."/>