qemu: checkpoint: Don't merge checkpoints during deletion

qemu: checkpoint: Don't merge checkpoints during deletion

Now that we've switched to the simple handling, the first thing that can
be massively simplified is checkpoint deletion. We now need to only go
through the backing chain and find the appropriately named bitmaps and
delete them, no complex lookups or merging.

Note that compared to other functions this deletes the bitmap in all
layers compared to others where we expect only exactly 1 bitmap of a
name in the backing chain to prevent potential problems.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>