Remove legacy-centos-7-3-node nodeset

We have no jobs using them, lets also delete it.

Change-Id: I4e653ee1cc4b9c2594231a7c2330bdbcaf16c177
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul.d/nodesets.yaml

@@ -173,21 +173,6 @@
         nodes:
           - secondary
 
-- nodeset:
-    name: legacy-centos-7-3-node
-    nodes:
-      - name: primary
-        label: centos-7
-      - name: secondary-1
-        label: centos-7
-      - name: secondary-2
-        label: centos-7
-    groups:
-      - name: subnodes
-        nodes:
-          - secondary-1
-          - secondary-2
-
 - nodeset:
     name: legacy-ubuntu-xenial-2-node
     nodes: