f2fs: remove unnecessary struct declaration

struct dnode_of_data is defined at 897th line.
The declaration here is unnecessary. Remove it.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -3311,7 +3311,6 @@ void f2fs_hash_filename(const struct inode *dir, struct f2fs_filename *fname);
 /*
  * node.c
  */
-struct dnode_of_data;
 struct node_info;
 
 int f2fs_check_nid_range(struct f2fs_sb_info *sbi, nid_t nid);