Fix compiler warnings. (#8022)

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

tests/cpp/common/test_quantile.cu

@@ -79,6 +79,7 @@ TEST(GPUQuantile, Unique) {
 // if with_error is true, the test tolerates floating point error
 void TestQuantileElemRank(int32_t device, Span<SketchEntry const> in,
                           Span<bst_row_t const> d_columns_ptr, bool with_error = false) {
+  dh::safe_cuda(cudaSetDevice(device));
   std::vector<SketchEntry> h_in(in.size());
   dh::CopyDeviceSpanToVector(&h_in, in);
   std::vector<bst_row_t> h_columns_ptr(d_columns_ptr.size());
@@ -478,7 +479,7 @@ TEST(GPUQuantile, SameOnAllWorkers) {
     dh::CopyDeviceSpanToVector(&h_base_line, base_line);
 
     size_t offset = 0;
-    for (size_t i = 0; i < world; ++i) {
+    for (decltype(world) i = 0; i < world; ++i) {
       auto comp = dh::ToSpan(all_workers).subspan(offset, size_as_float);
       std::vector<float> h_comp(comp.size());
       dh::CopyDeviceSpanToVector(&h_comp, comp);