Remove a check for READ_VERSION==0 in BlockStructureBlockData._add_transformer.

Signed-off-by: Tomasz Gargas <tomasz@opencraft.com>

openedx/core/djangoapps/content/block_structure/block_structure.py

@@ -730,7 +730,7 @@ class BlockStructureBlockData(BlockStructure):
         Adds the given transformer to the block structure by recording
         its current version number.
         """
-        if transformer.READ_VERSION == 0 or transformer.WRITE_VERSION == 0:
+        if transformer.WRITE_VERSION == 0:
             raise TransformerException('Version attributes are not set on transformer {0}.', transformer.name())
         self.set_transformer_data(transformer, TRANSFORMER_VERSION_KEY, transformer.WRITE_VERSION)