Switch jobs to python3

- ironic-base is python3 by default, we don't need `USE_PYTHON3`

Depends-On: https://review.opendev.org/#/c/687521/
Change-Id: I75ee04332e11737c3fa4fb429a9a306a6b05b011

zuul.d/ironicclient-jobs.yaml

@@ -35,6 +35,7 @@
         mysql: True
         rabbit: True
 
+# NOTE(iurygregory): drop when the time comes
 - job:
     name: ironicclient-functional-python2
     parent: ironicclient-functional
@@ -52,9 +53,9 @@
     timeout: 10800
     vars:
       devstack_localrc:
-        USE_PYTHON3: True
         EBTABLES_RACE_FIX: True
 
+# NOTE(iurygregory): drop when the time comes
 - job:
     name: ironicclient-tempest-python2
     parent: ironicclient-tempest