Remove legacy-centos-7-2-node nodeset

According to codesearch.o.o we have no jobs using this nodeset, remove
it.

Change-Id: I0f47ade2495e78c17222dc1dde71d7b9662bfef2
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul.d/nodesets.yaml

@@ -161,18 +161,6 @@
       - name: primary
         label: ubuntu-xenial
 
-- nodeset:
-    name: legacy-centos-7-2-node
-    nodes:
-      - name: primary
-        label: centos-7
-      - name: secondary
-        label: centos-7
-    groups:
-      - name: subnodes
-        nodes:
-          - secondary
-
 - nodeset:
     name: legacy-ubuntu-xenial-2-node
     nodes: