[mesh-forwarder] do not log dropping of received beacon frames (#2773)

This change removes the log message indicating dropping of a
received beacon frame.

src/core/thread/mesh_forwarder.cpp

@@ -1169,6 +1169,9 @@ void MeshForwarder::HandleReceivedFrame(Mac::Frame &aFrame)
 
 #endif
 
+    case Mac::Frame::kFcfFrameBeacon:
+        break;
+
     default:
         error = OT_ERROR_DROP;
         break;