Use BLKPG_DEL_PARTITION when deleting partitions instead of libparted

Use BLKPG_DEL_PARTITION when deleting partitions instead of libparted

Sadly, libparted gets this wrong since it does the equivalent of
'blockdev --rereadpt' which fails if one or more partitions are busy.

Also disallow deleting an extended partition if logical partitions
exist. We can undo this if we add some code to also do DEL_PARTITION
for the logical partitions (including scrubbing signatures).