Fix compiler warnings. (#8022)

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

src/data/simple_dmatrix.cu

@@ -15,7 +15,7 @@ namespace data {
 // Current implementation assumes a single batch. More batches can
 // be supported in future. Does not currently support inferring row/column size
 template <typename AdapterT>
-SimpleDMatrix::SimpleDMatrix(AdapterT* adapter, float missing, int nthread) {
+SimpleDMatrix::SimpleDMatrix(AdapterT* adapter, float missing, int32_t /*nthread*/) {
   auto device = (adapter->DeviceIdx() < 0 || adapter->NumRows() == 0) ? dh::CurrentDevice()
                                                                       : adapter->DeviceIdx();
   CHECK_GE(device, 0);