gnu: Remove obsolete variable.

* gnu/packages/python-xyz.scm (python-packaging-next): Remove variable.
* gnu/packages/openstack.scm (python-oslo.utils)[propagated-inputs]: Change
from PYTHON-PACKAGING-NEXT to PYTHON-PACKAGING.

gnu/packages/openstack.scm

@@ -769,7 +769,7 @@ for debugging, and better support for mocking results.")
             python-netaddr
             python-netifaces
             python-pbr
-            python-packaging-next
+            python-packaging
             python-pyparsing
             python-pytz))
     (native-inputs