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/metric/test_elementwise_metric.cc

@@ -36,9 +36,9 @@ inline void CheckDeterministicMetricElementWise(StringView name, int32_t device)
     h_labels[i] = dist(&lcg);
   }
 
-  auto result = metric->Eval(predts, info, false);
+  auto result = metric->Eval(predts, info);
   for (size_t i = 0; i < 8; ++i) {
-    ASSERT_EQ(metric->Eval(predts, info, false), result);
+    ASSERT_EQ(metric->Eval(predts, info), result);
   }
 }
 }  // anonymous namespace