Mariabackup: Remove the --stats option

The option was basically duplicating InnoDB functionality.
Persistent statistics can be accessed via the tables
mysql.innodb_table_stats and mysql.innodb_index_stats.

extra/mariabackup/encryption_plugin.cc

@@ -126,7 +126,6 @@ void encryption_plugin_prepare_init(int argc, char **argv)
 
   if (!xb_plugin_load)
   {
-    /* This prevents crashes e.g in --stats with wrong my.cnf*/
     finalize_encryption_plugin(0);
     return;
   }

extra/mariabackup/xb0xb.h

@@ -44,19 +44,6 @@ dberr_t*	err,	/*!< out: this is set to DB_ERROR if an error
                 os_file_dir_t	dir,	/*!< in: directory stream */
                 os_file_stat_t*	info)	/*!< in/out: buffer where the
                                       info is returned */;
-buf_block_t* btr_node_ptr_get_child(
-  const rec_t*	node_ptr,/*!< in: node pointer */
-  dict_index_t*	index,	/*!< in: index */
-  const ulint*	offsets,/*!< in: array returned by rec_get_offsets() */
-  mtr_t*		mtr)	/*!< in: mtr */;
-
-buf_block_t*
-btr_root_block_get(
-/*===============*/
-const dict_index_t*	index,	/*!< in: index tree */
-ulint			mode,	/*!< in: either RW_S_LATCH
-                or RW_X_LATCH */
-                mtr_t*			mtr)	/*!< in: mtr */;
 fil_space_t*
 fil_space_get_by_name(const char *);
 ibool