Zuul: do not use USE_PYTHON3

Devstack has removed the USE_PYTHON3 variable[1][2] and now always uses
Python 3.

[1] https://review.opendev.org/c/openstack/devstack/+/920658
[2] Commit 5412dbfe7b797149f1f68100de8003b1876398fe

Change-Id: I184a3aaefff83e2ef9dd0a84b1dc9411a12fb34b

Author: CyrilRoelandteNovance

zuul.d/base.yaml

@@ -81,7 +81,6 @@
         Q_ML2_PLUGIN_MECHANISM_DRIVERS: ovn
         Q_ML2_PLUGIN_TYPE_DRIVERS: local,flat,vlan,geneve
         Q_ML2_TENANT_NETWORK_TYPE: geneve
-        USE_PYTHON3: True
         TEMPEST_PLUGINS: '/opt/stack/octavia-tempest-plugin'
         OCTAVIA_NODE: api
         OCTAVIA_TEMPEST_PLUGIN_CUSTOMIZE_IMAGE: true