[breaking] [jvm-packages] Remove rabit check point. (#9599)

- Add `numBoostedRound` to jvm packages
- Remove rabit checkpoint version.
- Change the starting version of training continuation in JVM [breaking].
- Redefine the checkpoint version policy in jvm package. [breaking]
- Rename the Python check point callback parameter. [breaking]
- Unifies the checkpoint policy between Python and JVM.

include/xgboost/c_api.h

@@ -1308,24 +1308,6 @@ XGB_DLL int XGBoosterSerializeToBuffer(BoosterHandle handle, bst_ulong *out_len,
 XGB_DLL int XGBoosterUnserializeFromBuffer(BoosterHandle handle,
                                            const void *buf, bst_ulong len);
 
-/*!
- * \brief Initialize the booster from rabit checkpoint.
- *  This is used in distributed training API.
- * \param handle handle
- * \param version The output version of the model.
- * \return 0 when success, -1 when failure happens
- */
-XGB_DLL int XGBoosterLoadRabitCheckpoint(BoosterHandle handle,
-                                         int* version);
-
-/*!
- * \brief Save the current checkpoint to rabit.
- * \param handle handle
- * \return 0 when success, -1 when failure happens
- */
-XGB_DLL int XGBoosterSaveRabitCheckpoint(BoosterHandle handle);
-
-
 /*!
  * \brief Save XGBoost's internal configuration into a JSON document.  Currently the
  *        support is experimental, function signature may change in the future without