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/common/test_partition_builder.cc

@@ -50,8 +50,8 @@ TEST(PartitionBuilder, BasicTest) {
         right[i] = left_total + value_right++;
       }
 
-      builder.SetNLeftElems(nid, begin, end, n_left);
-      builder.SetNRightElems(nid, begin, end, n_right);
+      builder.SetNLeftElems(nid, begin, n_left);
+      builder.SetNRightElems(nid, begin, n_right);
     }
   }
   builder.CalculateRowOffsets();