Remove legacy-centos-7-4-node nodeset

We have no jobs using this nodeset, lets remove it.

Change-Id: Ib5a70c1a71a3d83e504b5c6caaf60c4e71bc6e68
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul.d/nodesets.yaml

@@ -188,24 +188,6 @@
           - secondary-1
           - secondary-2
 
-- nodeset:
-    name: legacy-centos-7-4-node
-    nodes:
-      - name: primary
-        label: centos-7
-      - name: secondary-1
-        label: centos-7
-      - name: secondary-2
-        label: centos-7
-      - name: secondary-3
-        label: centos-7
-    groups:
-      - name: subnodes
-        nodes:
-          - secondary-1
-          - secondary-2
-          - secondary-3
-
 - nodeset:
     name: legacy-ubuntu-xenial-2-node
     nodes: