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: Idca9a586580f74502ba05eaf2340aa20606515b6

.zuul.yaml

@@ -9,8 +9,6 @@
       - openstack/devstack
       - openstack/devstack-plugin-container
     vars:
-      devstack_localrc:
-        USE_PYTHON3: true
       devstack_plugins:
         devstack-plugin-container: https://opendev.org/openstack/devstack-plugin-container
 
@@ -34,7 +32,6 @@
         k8s-master: true
       devstack_localrc:
         K8S_TOKEN: "9agf12.zsu5uh2m4pzt3qba"
-        USE_PYTHON3: true
       devstack_plugins:
         devstack-plugin-container: https://opendev.org/openstack/devstack-plugin-container
     group-vars:
@@ -46,7 +43,6 @@
           k8s-node: true
         devstack_localrc:
           K8S_TOKEN: "9agf12.zsu5uh2m4pzt3qba"
-          USE_PYTHON3: true
 
 - project:
     check: