Removed unnecessary cast

Summary: Floating-point scale conversion doesn't require a cast

Reviewed By: yfeldblum

Differential Revision: D18052505

fbshipit-source-id: 3d87a916da5cec03b23041908066a9862fab6fde

folly/TokenBucket.h

@@ -106,9 +106,8 @@ class BasicDynamicTokenBucket {
    * Returns the current time in seconds since Epoch.
    */
   static double defaultClockNow() noexcept {
-    using dur = std::chrono::duration<double>;
     auto const now = Clock::now().time_since_epoch();
-    return std::chrono::duration_cast<dur>(now).count();
+    return std::chrono::duration<double>(now).count();
   }
 
   /**