[3052] Removed extraneous const function return type.

Debian complained about const Enum function declaration in
D2QueueMgr.

src/bin/d2/d2_queue_mgr.h

@@ -248,7 +248,7 @@ public:
     void setMaxQueueSize(const size_t max_queue_size);
 
     /// @brief Returns the current state.
-    const State getMgrState() const {
+    State getMgrState() const {
         return (mgr_state_);
     }