Remove Download.FeedbackDialogEnabled histogram

This is not used anywhere

BUG=1088729

Change-Id: I3831bce56c5817ef619c6b040f31f87e95622135
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225498Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774298}

tools/metrics/histograms/histograms.xml

@@ -43669,6 +43669,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Download.FeedbackDialogEnabled" enum="BooleanEnabled"
     expires_after="M85">
+  <obsolete>
+    Removed in 06/2020.
+  </obsolete>
   <owner>dtrainor@chromium.org</owner>
   <summary>
     Whether the user enables dangerous download feedback reporting after viewing