Remove centos-7-3-node / centos-7-4-node

It seems no jobs are configure to use these nodesets. Lets remove them
for now, and if we need them again, we can add them back.

Change-Id: Ibeedf070abd6754e8c3a534e43cee2901bcef40a
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul.d/nodesets.yaml

@@ -77,39 +77,6 @@
         nodes:
           - secondary
 
-- nodeset:
-    name: 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: 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: ubuntu-xenial-2-node
     nodes: