Drop the opensuse-15 nodeset

We are dropping support for opensuse leap 15 test environments because
our current leap 15 env is based on 15.2 which is quite ancient at this
point. Additionally leap as a distro appears to be going away and will
be replaced with some other dev type distro in that ecosystem.

Note the depends on below are a subset of the removals from repos but
they are the ones that I think we should get done before we land this
change.

Depends-On: https://review.opendev.org/c/zuul/zuul-jobs/+/909772
Depends-On: https://review.opendev.org/c/opendev/bindep/+/909771
Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/909770
Depends-On: https://review.opendev.org/c/openstack/project-config/+/909775
Change-Id: Icda87007f38f4aa9f53010c45fdd070ed646165d

zuul.d/nodesets.yaml

@@ -77,12 +77,6 @@
       - name: gentoo-17-0-systemd
         label: gentoo-17-0-systemd
 
-- nodeset:
-    name: opensuse-15
-    nodes:
-      - name: opensuse-15
-        label: opensuse-15
-
 - nodeset:
     name: ubuntu-bionic
     nodes: