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: Ifa9f21383f31bae9abe24a9b47a1a0f50523b554
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

Author: CyrilRoelandteNovance

.zuul.yaml

@@ -59,13 +59,6 @@
     vars:
       tox_envlist: py3
 
-- job:
-    name: glanceclient-dsvm-functional-py3
-    parent: glanceclient-dsvm-functional
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: true
-
 - project:
     templates:
       - check-requirements
@@ -98,6 +91,3 @@
             branches: master
         - glanceclient-tox-py3-oslo-tips:
             branches: master
-    experimental:
-      jobs:
-        - glanceclient-dsvm-functional-py3