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.

src/common/quantile.h

@@ -774,7 +774,7 @@ class HostSketchContainer : public SketchContainerImpl<WQuantileSketch<float, fl
 
  public:
   HostSketchContainer(int32_t max_bins, MetaInfo const &info, std::vector<size_t> columns_size,
-                      bool use_group, Span<float const> hessian, int32_t n_threads);
+                      bool use_group, int32_t n_threads);
 };
 
 /**
@@ -868,7 +868,7 @@ class SortedSketchContainer : public SketchContainerImpl<WXQuantileSketch<float,
  public:
   explicit SortedSketchContainer(int32_t max_bins, MetaInfo const &info,
                                  std::vector<size_t> columns_size, bool use_group,
-                                 Span<float const> hessian, int32_t n_threads)
+                                 int32_t n_threads)
       : SketchContainerImpl{columns_size, max_bins, info.feature_types.ConstHostSpan(), use_group,
                             n_threads} {
     monitor_.Init(__func__);