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: I7484333bd522f97fdb88f9f68e8726ec6620b40b

.zuul.yaml

@@ -21,7 +21,6 @@
         MYSQL_ROOT_PW: secretdatabase
         MISTRAL_USE_MOD_WSGI: true
         MISTRAL_RPC_IMPLEMENTATION: oslo
-        USE_PYTHON3: true
       devstack_services:
         heat: true
         h-api: true