Previous patch naively removed all code relevant to disk format
checking. The semantics now dictate that the format check when creating
external snapshots is now impossible as we always fill in the format for
disks in domain definition in the post-parse callback.

Remove the impossible code path.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>