btrfs: remove the btrfs_item_end() helper

We're only using btrfs_item_end() from btrfs_item_end_nr(), so this can
be collapsed.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>