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: Ia42a03bd73b1b4b6e157c9ce2d7e02d1d59790ba
diff --git a/zuul.d/manila-tempest-jobs.yaml b/zuul.d/manila-tempest-jobs.yaml
index 36a1faf..8952b88 100644
--- a/zuul.d/manila-tempest-jobs.yaml
+++ b/zuul.d/manila-tempest-jobs.yaml
@@ -38,7 +38,6 @@
         horizon: false
         tls-proxy: true
       devstack_localrc:
-        USE_PYTHON3: true
         MYSQL_REDUCE_MEMORY: True
         MANILA_USE_DOWNGRADE_MIGRATIONS: true
         MANILA_INSTALL_TEMPEST_PLUGIN_SYSTEMWIDE: false
@@ -102,7 +101,6 @@
         yaml: true
         yml: true
       devstack_localrc:
-        USE_PYTHON3: true
         MANILA_INSTALL_TEMPEST_PLUGIN_SYSTEMWIDE: false
         MANILA_SERVICE_IMAGE_ENABLED: false
         MANILA_SHARE_MIGRATION_PERIOD_TASK_INTERVAL: 1