zuul: remove nodesets

This is a follow up to the previous commit -- now that there are no
longer any jobs that use these nodesets we can remove them here.

Change-Id: I84b5554184cfdbc006ed970559c4cb3e47a02768

.zuul.d/nodesets.yaml

@@ -1,50 +0,0 @@
-# Nodeset used to test instances of ARA API deployed on different operating
-# systems against MySQL and PostgreSQL simultaneously.
-- nodeset:
-    name: ara-database-server-multinode
-    nodes:
-      - name: database-server
-        label: centos-8
-      - name: ubuntu-bionic
-        label: ubuntu-bionic
-      - name: fedora-32
-        label: fedora-32
-      - name: centos-8
-        label: centos-8
-    groups:
-      - name: ara-database-server
-        nodes:
-          - database-server
-      - name: ara-api-server
-        nodes:
-          - ubuntu-bionic
-          - fedora-32
-          - centos-8
-
-# Nodeset used to test instances of ARA API deployed on different operating
-# systems simultaneously.
-- nodeset:
-    name: ara-multinode
-    nodes:
-      - name: ubuntu-bionic
-        label: ubuntu-bionic
-      - name: fedora-32
-        label: fedora-32
-      - name: centos-8
-        label: centos-8
-    groups:
-      - name: ara-api-server
-        nodes:
-          - ubuntu-bionic
-          - fedora-32
-          - centos-8
-
-- nodeset:
-    name: ara-fedora-32
-    nodes:
-      - name: fedora-32
-        label: fedora-32
-    groups:
-      - name: ara-api-server
-        nodes:
-          - fedora-32