Remove failed msg for version attribute

Author: bobleesj

tests/test_version.py

@@ -6,5 +6,5 @@
 
 def test_package_version():
     """Ensure the package version is defined and not set to the initial placeholder."""
-    assert hasattr(diffpy.structure, '__version__'), "The package version is not defined."
+    assert hasattr(diffpy.structure, '__version__')
     assert diffpy.structure.__version__ != "0.0.0"