Fix compiler warnings. (#8022)

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

tests/cpp/tree/gpu_hist/test_row_partitioner.cu

@@ -119,7 +119,7 @@ void TestFinalise() {
     rp.FinalisePosition(
         &ctx, task, &position,
         [=] __device__(RowPartitioner::RowIndexT ridx, int position) { return 7; },
-        [] XGBOOST_DEVICE(size_t idx) { return false; });
+        [] XGBOOST_DEVICE(size_t) { return false; });
 
     auto position = rp.GetPositionHost();
     for (auto p : position) {