Disable opensuse-15 on stein

There is almost no interested in stein on openSUSE Leap remaining that I
know of, so stop testing it. It won't work on 15.2 with python2. On 15.1
it was working with https://review.opendev.org/#/c/735640/ , but
maintaining 15.1 in the CI is probably not worth it.

In https://review.opendev.org/#/c/745252/ the image of openSUSE Leap used
by Zuul was updated to 15.2. It ships with libvirt 6.0.0 instead of
5.1.0, but the old version of libvirt-python doesn't build/install with
the newer libvirt. The newer libvirt-python 6.0.0 doesn't work with
python2, but requires python3, so updating the upper constraints is not
easy.

Change-Id: Iebaadee46fa8c3cae6cf7cd9f15565281e030bd5

.zuul.yaml

@@ -619,7 +619,6 @@
         - devstack-ipv6:
             voting: false
         - devstack-platform-centos-7
-        - devstack-platform-opensuse-15
         - devstack-platform-fedora-latest
         - devstack-platform-xenial
         - devstack-multinode