Simplify reinsert policy interface

Summary: Simplify the interface as the touch(), remove() methods are not needed now.

Reviewed By: therealgymmy

Differential Revision: D22547658

fbshipit-source-id: 6d5c799fb21cb4cb6ee20203c820f4d6ff7e8255

folly/stats/TDigest.h

@@ -113,7 +113,7 @@ class TDigest {
   double estimateQuantile(double q) const;
 
   double mean() const {
-    return count_ ? sum_ / count_ : 0;
+    return count_ > 0 ? sum_ / count_ : 0;
   }
 
   double sum() const {