Drop debian-buster nodesets

We are removing this node type and image from Nodepool and our mirrors
entirely. Buster has been replaced by bullseye and bookworm and we don't
need to keep an old largely unused debian release around.

Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/910008
Depends-On: https://review.opendev.org/c/openstack/project-config/+/910011
Depends-On: https://review.opendev.org/c/opendev/system-config/+/909787
Depends-On: https://review.opendev.org/c/zuul/zuul-jobs/+/909786
Change-Id: Id15101787af3284de352e7d3887218010f0bf80a

zuul.d/nodesets.yaml

@@ -47,18 +47,6 @@
       - name: debian-bookworm-arm64
         label: debian-bookworm-arm64
 
-- nodeset:
-    name: debian-buster
-    nodes:
-      - name: debian-buster
-        label: debian-buster
-
-- nodeset:
-    name: debian-buster-arm64
-    nodes:
-      - name: debian-buster-arm64
-        label: debian-buster-arm64
-
 - nodeset:
     name: debian-bullseye
     nodes: