bash-completion: stop being clever when listing block devices

bash-completion: stop being clever when listing block devices

Various commands such as blkid, cfdisk, fdisk, delpart, and so on listed
only partitions and missed for example disks and volume groups.  The
right thing to do is to list all block devices in all for all commands
performing operations with them.  This might occasionally list unexpected
devices that I think is lesser bad than missing some.

Addresses: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=764488
Signed-off-by: Sami Kerola <kerolasa@iki.fi>