Revert "storageVolCreateXMLFrom: Check if backend knows how to createVol"

This reverts commit 611a278f.

According to the original commit message, this is dead code:

  It is highly unlikely that a backend will know how to create a
  volume from a different volume (buildVolFrom) and not know how to
  create an empty volume (createVol).

src/storage/storage_driver.c

@@ -2037,13 +2037,6 @@ storageVolCreateXMLFrom(virStoragePoolPtr obj,
     if (newvol->target.capacity < origvol->target.capacity)
         newvol->target.capacity = origvol->target.capacity;
 
-    if (!backend->createVol) {
-        virReportError(VIR_ERR_NO_SUPPORT,
-                       "%s", _("storage pool does not support volume "
-                               "creation"));
-        goto cleanup;
-    }
-
     if (!backend->buildVolFrom) {
         virReportError(VIR_ERR_NO_SUPPORT,
                        "%s", _("storage pool does not support"