Optimize prediction with QuantileDMatrix. (#9096)

- Reduce overhead in `FVecDrop`.
- Reduce overhead caused by `HostVector()` calls.

src/tree/updater_refresh.cc

@@ -79,7 +79,7 @@ class TreeRefresher : public TreeUpdater {
                      dmlc::BeginPtr(stemp[tid]) + offset);
             offset += tree->NumNodes();
           }
-          feats.Drop(inst);
+          feats.Drop();
         });
       }
       // aggregate the statistics