rbd: rename deleteImage() to Delete()

Renaming the `Delete()` function makes it easier to implement the
upcoming `Volume` interface.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

internal/rbd/migration.go

@@ -82,7 +82,7 @@ func deleteMigratedVolume(ctx context.Context, parsedMigHandle *migrationVolID,
 		return err
 	}
 	defer rv.Destroy(ctx)
-	err = rv.deleteImage(ctx)
+	err = rv.Delete(ctx)
 	if err != nil {
 		log.ErrorLog(ctx, "failed to delete rbd image: %s, err: %v", rv, err)
 	}