Remove centos-7 nodepool image builds

This is the last step in cleaning centos-7 out of nodepool. The previous
change will have cleaned up uploads and now we can stop building the
images entirely.

Change-Id: Ie81d6d516cd6cd42ae9797025a39521ceede7b71

nodepool/nb04.opendev.org.yaml

@@ -137,9 +137,6 @@ diskimages:
 
 ############ CentOS and Fedora (and derivatives) #############
 
-  - name: centos-7
-    elements: []
-
   - name: centos-8-stream
     elements: []
 

nodepool/nl01.opendev.org.yaml

@@ -170,7 +170,6 @@ providers:
         labels: *provider_pools_labels
 
 diskimages:
-  - name: centos-7
   - name: centos-8-stream
   - name: centos-9-stream
   - name: debian-bookworm

nodepool/nl02.opendev.org.yaml

@@ -140,7 +140,6 @@ providers:
             key-name: infra-root-keys-2020-05-13
 
 diskimages:
-  - name: centos-7
   - name: centos-8-stream
   - name: centos-9-stream
   - name: debian-bookworm

nodepool/nl03.opendev.org.yaml

@@ -474,7 +474,6 @@ providers:
             key-name: infra-root-keys-2020-05-13
 
 diskimages:
-  - name: centos-7
   - name: centos-8-stream
   - name: centos-9-stream
   - name: centos-8-stream-arm64

nodepool/nl04.opendev.org.yaml

@@ -194,7 +194,6 @@ providers:
         labels: *provider_pools_labels
 
 diskimages:
-  - name: centos-7
   - name: centos-8-stream
   - name: centos-9-stream
   - name: debian-bookworm

nodepool/nodepool.yaml

@@ -158,16 +158,6 @@ diskimages:
 
 ############ CentOS and Fedora (and derivatives) #############
 
-  - name: centos-7
-    parent: base
-    pause: false
-    elements:
-      - centos-minimal
-      - epel
-      - cache-devstack
-    env-vars:
-      DIB_EPEL_DISABLED: '1'
-
   - name: centos-8-stream
     parent: base
     pause: false