Remove unused code. (#7175)

src/tree/gpu_hist/row_partitioner.cu

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2017-2019 XGBoost contributors
+ * Copyright 2017-2021 XGBoost contributors
  */
 #include <thrust/iterator/discard_iterator.h>
 #include <thrust/iterator/transform_output_iterator.h>
@@ -10,16 +10,6 @@
 
 namespace xgboost {
 namespace tree {
-
-struct IndicateLeftTransform {
-  bst_node_t left_nidx;
-  explicit IndicateLeftTransform(bst_node_t left_nidx) : left_nidx(left_nidx) {}
-  __host__ __device__ __forceinline__ size_t
-  operator()(const bst_node_t& x) const {
-    return x == left_nidx ? 1 : 0;
-  }
-};
-
 struct IndexFlagTuple {
   size_t idx;
   size_t flag;