Remove Python 3 variables from devstack config

The USE_PYTHON3 variable was removed earlier this year [1].

[1]  https://review.opendev.org/c/openstack/devstack/+/920658

Change-Id: I50d8c94e00c38d6ec818a99727b2829bd330fba0

Author: priteau

.zuul.yaml

@@ -21,8 +21,6 @@
         cloudkitty: https://opendev.org/openstack/cloudkitty
       devstack_localrc:
         CLOUDKITTY_FETCHER: keystone
-        DEVSTACK_GATE_USE_PYTHON3: "True"
-        USE_PYTHON3: True
       devstack_services:
         ck-api: true
         horizon: false