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: Id2932b40c810a92d0d1219e7b43fc1917e88de8a

zuul.d/rally.yaml

@@ -4,7 +4,6 @@
     timeout: 10800
     vars:
       devstack_localrc:
-        USE_PYTHON3: true
         OSPROFILER_COLLECTOR: redis
         OSPROFILER_HMAC_KEYS: "neutron-hmac-key-used-in-zuul-ci"
         Q_ML2_TENANT_NETWORK_TYPE: vxlan
@@ -178,7 +177,6 @@
         # is finished or the mech driver does not call monkey_patch().
         Q_ML2_PLUGIN_MECHANISM_DRIVERS: ovn
         Q_ML2_TENANT_NETWORK_TYPE: geneve
-        USE_PYTHON3: true
         PHYSICAL_NETWORK: public
         Q_USE_PROVIDERNET_FOR_PUBLIC: true
         ENABLE_CHASSIS_AS_GW: true

zuul.d/tempest-multinode.yaml

@@ -532,7 +532,6 @@
           PHYSICAL_NETWORK: public
           ENABLE_CHASSIS_AS_GW: false
           OVN_DBS_LOG_LEVEL: dbg
-          USE_PYTHON3: True
           ENABLE_TLS: True
 
 - job:

zuul.d/tempest-singlenode.yaml

@@ -497,7 +497,6 @@
         ENABLE_CHASSIS_AS_GW: true
         OVN_L3_CREATE_PUBLIC_NETWORK: true
         OVN_DBS_LOG_LEVEL: dbg
-        USE_PYTHON3: True
         BUILD_TIMEOUT: 784
         ENABLE_TLS: True
         OVN_IGMP_SNOOPING_ENABLE: True