Both `rbdVolume.repairImageID()` and `rbdVolume.storeImageID()` do the
same thing. `repairImageID()` includes optional extra checking for an
already set ImageID, which is not part of `storeImageID()`.
Remove some unneeded `ImageID == ""` checking because `repairImageID()`
does that, and remove the `storeImageID()` function.
Signed-off-by: Niels de Vos <ndevos@ibm.com>