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: Id42c2031e462f9104d59476e2914aebdd3af3475

elements/upstream-training/static/tmp/local.conf

@@ -13,15 +13,10 @@ SERVICE_PASSWORD=$ADMIN_PASSWORD
 USE_SCREEN=False
 USE_SYSTEMD=True
 
-# Times change...
-USE_PYTHON3=True
-
 # Git fine tuning. HTTPS is usually enabled on firewalls.
 # Depth should be only one for upstream training purposes
 GIT_DEPTH=1
 GIT_BASE=https://opendev.org
 
-USE_PYTHON3=True
-
 # Let's save some memory
 API_WORKERS=1