Fix compiler warnings. (#7974)

- Remove unused parameters. There are still many warnings that are not yet
addressed. Currently, the warnings in dmlc-core dominate the error log.
- Remove `distributed` parameter from metric.
- Fixes some warnings about signed comparison.

tests/cpp/helpers.h

@@ -296,9 +296,7 @@ class RandomDataGenerator {
                                            bool float_label = true,
                                            size_t classes = 1) const;
 #if defined(XGBOOST_USE_CUDA)
-  std::shared_ptr<DMatrix> GenerateDeviceDMatrix(bool with_label = false,
-                                                 bool float_label = true,
-                                                 size_t classes = 1);
+  std::shared_ptr<DMatrix> GenerateDeviceDMatrix();
 #endif
 };