Remove unreferenced function declarations.

datapagemap_create() and datapagemap_destroy() were declared extern,
but they don't actually exist anywhere.  Per YUriy Zhuravlev and
Michael Paquier.

src/bin/pg_rewind/datapagemap.h

@@ -22,8 +22,6 @@ struct datapagemap
 typedef struct datapagemap datapagemap_t;
 typedef struct datapagemap_iterator datapagemap_iterator_t;
 
-extern datapagemap_t *datapagemap_create(void);
-extern void datapagemap_destroy(datapagemap_t *map);
 extern void datapagemap_add(datapagemap_t *map, BlockNumber blkno);
 extern datapagemap_iterator_t *datapagemap_iterate(datapagemap_t *map);
 extern bool datapagemap_next(datapagemap_iterator_t *iter, BlockNumber *blkno);