Restrict access to cfg_ in gbm. (#4801)

* Restrict access to `cfg_` in gbm.

* Verify having correct updaters.

* Remove `grow_global_histmaker`

This updater is the same as `grow_histmaker`.  The former is not in our
document so we just remove it.

src/tree/updater_gpu_hist.cu

@@ -1460,7 +1460,7 @@ class GPUHistMaker : public TreeUpdater {
   }
 
   char const* Name() const override {
-    return "gpu_hist";
+    return "grow_gpu_hist";
   }
 
  private: