storage: Remove storageBackendGenerateSecretData
Since we no longer support creating qcow2 encryption format
volumes, we no longer have to possibly create some secret and
have no real need for the function, so move the remaining
functionality to build the secret path back into the caller
storageBackendCreateQemuImg.
Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>