btrfs: remove unused define EXTENT_SIZE_PER_ITEM

This was added  in c61a16a7 ("Btrfs: fix the confusion between
delalloc bytes and metadata bytes") and removed in 03fe78cc
("btrfs: use delalloc_bytes to determine flush amount for
shrink_delalloc") where the calculation was reworked to use a
non-constant numbers. This was found by 'make W=2'.
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -605,8 +605,6 @@ static inline u64 calc_reclaim_items_nr(const struct btrfs_fs_info *fs_info,
 	return nr;
 }
 
-#define EXTENT_SIZE_PER_ITEM	SZ_256K
-
 /*
  * shrink metadata reservation for delalloc
  */