btrfs: ulist: Add ulist_del() function.

This function will delete unode with given (val,aux) pair.
And with this patch, seqnum for debug usage doesn't have any meaning
now, so remove them.

This is used by later patches to skip snapshot root.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>