Use only Yoga tests

TC has decided to keep support for Python 3.6 during the Yoga cycle.
For more info see [1]

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-December/026164.html

Change-Id: I0ac128e386818dd03d64dfebf7f2c4a1e5ec3e28

.zuul.yaml

@@ -3,17 +3,3 @@
       - check-requirements
       - openstack-python3-yoga-jobs
       - publish-openstack-docs-pti
-    check:
-      jobs:
-        # NOTE(rpittau): yoga jobs support py38 and py39 but we want to keep
-        # testing py36 to maintain compatibility with older distributions like
-        # CentOS Stream 8, Ubuntu Bionic, Debian 10, and openSuse Leap
-        # Remove after Yoga
-        - openstack-tox-py36
-    gate:
-      jobs:
-        # NOTE(rpittau): yoga jobs support py38 and py39 but we want to keep
-        # testing py36 to maintain compatibility with older distributions like
-        # CentOS Stream 8, Ubuntu Bionic, Debian 10, and openSuse Leap
-        # Remove after Yoga
-        - openstack-tox-py36