Remove unused page size constant (#8457)

include/xgboost/data.h

@@ -619,9 +619,6 @@ class DMatrix {
                          int32_t nthread, std::string cache);
 
   virtual DMatrix *Slice(common::Span<int32_t const> ridxs) = 0;
-  /*! \brief Number of rows per page in external memory.  Approximately 100MB per page for
-   *  dataset with 100 features. */
-  static const size_t kPageSize = 32UL << 12UL;
 
  protected:
   virtual BatchSet<SparsePage> GetRowBatches() = 0;