Delete DownloadActionDelegate::SetDownloadStatus() method.

This method is only used by the caller to detect a programming error in
the DownloadAction (calling BytesReceived when not downloading) and log
a message. This patch removes the method from the delegate interface
and makes sure it doesn't issue a call to BytesReceived when not
activelly downloading.

Bug: 25773375
TEST=FEATURES=test emerge-link update_engine

Change-Id: I8ff5c53f1fd40c0777b3f6523703a8bee71c019d
diff --git a/update_attempter.h b/update_attempter.h
index b08b8a8..c0e8de0 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -148,8 +148,7 @@
   // UPDATED_NEED_REBOOT. Returns true on sucess, false otherwise.
   bool RebootIfNeeded();
 
-  // DownloadActionDelegate methods
-  void SetDownloadStatus(bool active) override;
+  // DownloadActionDelegate method.
   void BytesReceived(uint64_t bytes_received, uint64_t total) override;
 
   // Broadcasts the current status over D-Bus.
@@ -418,10 +417,6 @@
   brillo::MessageLoop::TaskId manage_shares_id_{
       brillo::MessageLoop::kTaskIdNull};
 
-  // Set to true if an update download is active (and BytesReceived
-  // will be called), set to false otherwise.
-  bool download_active_ = false;
-
   // For status:
   UpdateStatus status_{UpdateStatus::IDLE};
   double download_progress_ = 0.0;