btrfs: delayed-inode: drop pointless BUG_ON in __btrfs_remove_delayed_item()

There's a BUG_ON checking for a valid pointer of fs_info::delayed_root
but it is valid since init_mount_fs_info() and has the same lifetime as
fs_info.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>