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.cc

@@ -167,7 +167,7 @@ double GetMultiMetricEval(xgboost::Metric* metric,
   info.weights_.HostVector() = weights;
   info.group_ptr_ = groups;
 
-  return metric->Eval(preds, info, false);
+  return metric->Eval(preds, info);
 }
 
 namespace xgboost {
@@ -653,8 +653,6 @@ class RMMAllocator {};
 
 void DeleteRMMResource(RMMAllocator* r) {}
 
-RMMAllocatorPtr SetUpRMMResourceForCppTests(int argc, char** argv) {
-  return {nullptr, DeleteRMMResource};
-}
+RMMAllocatorPtr SetUpRMMResourceForCppTests(int, char**) { return {nullptr, DeleteRMMResource}; }
 #endif  // !defined(XGBOOST_USE_RMM) || XGBOOST_USE_RMM != 1
 }  // namespace xgboost