Fix compiler warnings. (#8022)

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

tests/cpp/tree/gpu_hist/test_histogram.cu

@@ -143,7 +143,6 @@ void TestGPUHistogramCategorical(size_t num_categories) {
 
   std::vector<GradientPairPrecise> h_cat_hist(cat_hist.size());
   thrust::copy(cat_hist.begin(), cat_hist.end(), h_cat_hist.begin());
-  auto cat_sum = std::accumulate(h_cat_hist.begin(), h_cat_hist.end(), GradientPairPrecise{});
 
   std::vector<GradientPairPrecise> h_encode_hist(encode_hist.size());
   thrust::copy(encode_hist.begin(), encode_hist.end(), h_encode_hist.begin());

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) {

tests/cpp/tree/hist/test_histogram.cc

@@ -181,8 +181,7 @@ void TestSyncHist(bool is_distributed) {
                                        starting_index, sync_count);
   } else {
     histogram.SyncHistogramLocal(&tree, nodes_for_explicit_hist_build_,
-                                 nodes_for_subtraction_trick_, starting_index,
-                                 sync_count);
+                                 nodes_for_subtraction_trick_);
   }
 
   using GHistRowT = common::GHistRow;