Fix and optimize logger (#4002)

* Fix logging switch statement.

* Remove debug_verbose_ in AllReducer.

* Don't construct the stream when not needed.

* Make default constructor deleted.

* Remove redundant IsVerbose.

src/tree/updater_gpu_hist.cu

@@ -971,9 +971,7 @@ class GPUHistMakerSpecialised{
       device_list_[index] = device_id;
     }
 
-    reducer_.Init(
-        device_list_,
-        ConsoleLogger::GlobalVerbosity() > ConsoleLogger::DefaultVerbosity());
+    reducer_.Init(device_list_);
 
     auto batch_iter = dmat->GetRowBatches().begin();
     const SparsePage& batch = *batch_iter;