core: Removed unused Disk and LunDisk c'tors

Non-default Disk c'tor is only used by LunDisk non-default c'tor, which
is not used at all.
Hence both are now removed, as are the now-useless default c'tors.

Change-Id: Ie1740ea1f198b75ef4cceed2ecd3fb8dc53c44c2
Signed-off-by: Vered Volansky <vvolansk@redhat.com>