nilfs2: deleted struct nilfs_dat_group_desc

struct nilfs_dat_group_desc is not used both in kernel and user spaces.
struct nilfs_palloc_group_desc is used instead.
Signed-off-by: NJiro SEKIBA <jir@unicus.jp>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

include/linux/nilfs2_fs.h

@@ -424,15 +424,6 @@ struct nilfs_dat_entry {
 	__le64 de_rsv;
 };
 
-/**
- * struct nilfs_dat_group_desc - block group descriptor
- * @dg_nfrees: number of free virtual block numbers in block group
- */
-struct nilfs_dat_group_desc {
-	__le32 dg_nfrees;
-};
-
-
 /**
  * struct nilfs_snapshot_list - snapshot list
  * @ssl_next: next checkpoint number on snapshot list