Remove USE_PYTHON3

Removed USE_PYTHON3: True bacause USE_PYTHON3 is True by default since
Ussuri release.

Author: hidekazuna

.zuul/playbooks/run.yaml

@@ -23,7 +23,6 @@
     - 'glance'
   - role: install-devstack
     environment:
-      USE_PYTHON3: 'True'
       DEVSTACK_GATE_LIBVIRT_TYPE: 'kvm'
       OS_BRANCH: 'stable/ussuri'
       OVERRIDE_ENABLED_SERVICES: 'dstat,etcd3,g-api,g-reg,key,mysql,n-api,n-api-meta,n-cond,n-cpu,n-novnc,n-sch,placement-api,q-agt,q-dhcp,q-l3,q-meta,q-metering,q-svc,rabbit'