Drop CentOS 7 jobs

All CentOS 7 jobs were replaced by CentOS 8 jobs during Ussuri cycle.
Now Train is EOL and none of open branches (including unmaintained ones)
run jobs using CentOS 7.

Change-Id: I82c251836fc1f12351f05b80dd9550953d65c493
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index d43876b..cb40790 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -29,16 +29,6 @@
           - controller
 
 - nodeset:
-    name: octavia-single-node-centos-7
-    nodes:
-      - name: controller
-        label: nested-virt-centos-7
-    groups:
-      - name: tempest
-        nodes:
-          - controller
-
-- nodeset:
     name: octavia-single-node-centos-8
     nodes:
       - name: controller
@@ -762,16 +752,6 @@
                 loadbalancer_topology: ACTIVE_STANDBY
 
 - job:
-    name: octavia-v2-dsvm-py2-scenario-centos-7
-    parent: octavia-v2-dsvm-py2-scenario
-    nodeset: octavia-single-node-centos-7
-    vars:
-      devstack_localrc:
-        OCTAVIA_AMP_BASE_OS: centos
-        OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID: 7
-        OCTAVIA_AMP_IMAGE_SIZE: 3
-
-- job:
     name: octavia-v2-dsvm-scenario-centos-8
     parent: octavia-v2-dsvm-scenario
     nodeset: octavia-single-node-centos-8
@@ -997,11 +977,6 @@
     parent: octavia-v2-dsvm-scenario
 
 - job:
-    name: octavia-v2-dsvm-scenario-centos-7
-    parent: octavia-v2-dsvm-py2-scenario-centos-7
-    nodeset: octavia-single-node-centos-7
-
-- job:
     name: octavia-v2-act-stdby-iptables-dsvm-scenario
     parent: octavia-dsvm-live-base
     pre-run: playbooks/act_stby_iptables/pre.yaml
@@ -1039,22 +1014,6 @@
         override-checkout: 2.30.0
 
 - job:
-    name: octavia-v2-act-stdby-iptables-dsvm-py2-scenario-centos-7
-    parent: octavia-v2-act-stdby-iptables-dsvm-py2-scenario
-    nodeset: octavia-single-node-centos-7
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: False
-        OCTAVIA_AMP_BASE_OS: centos
-        OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID: 7
-        OCTAVIA_AMP_IMAGE_SIZE: 3
-      devstack_local_conf:
-        test-config:
-          "$TEMPEST_CONFIG":
-            load_balancer:
-              amphora_ssh_user: centos
-
-- job:
     name: octavia-v2-act-stdby-dsvm-scenario-base
     parent: octavia-dsvm-live-base
     vars: