Fix compiler warnings. (#8022)

- Remove/fix unused parameters
- Remove deprecated code in rabit.
- Update dmlc-core.

src/tree/updater_sync.cc

@@ -33,7 +33,7 @@ class TreeSyncher : public TreeUpdater {
   }
 
   void Update(HostDeviceVector<GradientPair>*, DMatrix*,
-              common::Span<HostDeviceVector<bst_node_t>> out_position,
+              common::Span<HostDeviceVector<bst_node_t>> /*out_position*/,
               const std::vector<RegTree*>& trees) override {
     if (rabit::GetWorldSize() == 1) return;
     std::string s_model;