Remove update prediction cache from predictors. (#5312)

Move this function into gbtree, and uses only updater for doing so. As now the predictor knows exactly how many trees to predict, there's no need for it to update the prediction cache.

include/xgboost/c_api.h

@@ -531,6 +531,7 @@ XGB_DLL int XGBoosterSaveRabitCheckpoint(BoosterHandle handle);
  *        notice.
  *
  * \param handle handle to Booster object.
+ * \param out_len length of output string
  * \param out_str A valid pointer to array of characters.  The characters array is
  *                allocated and managed by XGBoost, while pointer to that array needs to
  *                be managed by caller.