dm: Remove unused declaration dm_get_rq_mapinfo()

Commit ae6ad75e ("dm: remove unused dm_get_rq_mapinfo()")
removed the implementation but leave declaration.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

include/linux/device-mapper.h

@@ -524,7 +524,6 @@ int dm_post_suspending(struct dm_target *ti);
 int dm_noflush_suspending(struct dm_target *ti);
 void dm_accept_partial_bio(struct bio *bio, unsigned int n_sectors);
 void dm_submit_bio_remap(struct bio *clone, struct bio *tgt_clone);
-union map_info *dm_get_rq_mapinfo(struct request *rq);
 
 #ifdef CONFIG_BLK_DEV_ZONED
 struct dm_report_zones_args {