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: I49b9d4ee6e88f24209cb7ff6fdf189c6361abfc5

.zuul.yaml

@@ -63,7 +63,6 @@
           DEST: "{{ devstack_bases.new }}"
           LOGFILE: "{{ devstack_bases.new }}/logs/devstacklog.txt"
         shared:
-          USE_PYTHON3: true
           # Common settings for OpenStack CI
           DATABASE_PASSWORD: secretdatabase
           # FIXME: temporarily define MYSQL_PASSWORD too,
@@ -336,10 +335,6 @@
     parent: grenade
     description: |
       Grenade job based on Python 3
-    vars:
-      grenade_devstack_localrc:
-        shared:
-          USE_PYTHON3: true
 
 - job:
     name: grenade-forward