Remove TEMPEST_BRANCH var from neutron queens job.

In devstack install_Tempest, ref is taken by TEMPEST_BRANCH var[1]. TEMPEST_BRANCH var on zuul job
definitoin is priority than the default value in devstack[2]. When we set the queens compatible Tempest
tag[1] (with review style not by tag name) on devstack stable/queens, that is not taken by devstack
because neutron queens job define the smae with different value[4].

Also Tag name (for example queens-em) never work because of devstack logic. Tempest tag name only work
if RECLONE is true otherwise it should be gerrit ref style. So always Tempest master is installed
on queens jobs.

Let's remove the TEMPEST_BRANCH from neutron jobs and rely on devstack deafult value.

CLoses-Bug: 1859988
[1] https://github.com/openstack/devstack/blob/stable/queens/lib/tempest#L658
[2] 786c485cf6/stackrc (L313)
[3] https://review.opendev.org/#/c/703679/
[4] https://zuul.opendev.org/t/openstack/build/0cc32b278afe4e51b787d87d480f7092/log/job-output.txt#12370

Change-Id: I07beb6e7bdcbaf7ffe72d47f9edb90df40641e68

.zuul.yaml

@@ -243,7 +243,6 @@
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_tempest) | join(',') }}"
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
-        TEMPEST_BRANCH: queens-em
 
 - job:
     name: neutron-tempest-plugin-api-rocky
@@ -550,7 +549,6 @@
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
-        TEMPEST_BRANCH: queens-em
 
 - job:
     name: neutron-tempest-plugin-scenario-openvswitch-rocky
@@ -701,7 +699,6 @@
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
-        TEMPEST_BRANCH: queens-em
       devstack_local_conf:
         test-config:
           # NOTE: ignores linux bridge's trunk delete on bound port test
@@ -910,7 +907,6 @@
       devstack_localrc:
         USE_PYTHON3: false
         TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
-        TEMPEST_BRANCH: queens-em
 
 - job:
     name: neutron-tempest-plugin-dvr-multinode-scenario-rocky
@@ -1004,7 +1000,6 @@
       devstack_localrc:
         USE_PYTHON3: false
         TEMPEST_PLUGINS: '"/opt/stack/designate-tempest-plugin /opt/stack/neutron-tempest-plugin"'
-        TEMPEST_BRANCH: queens-em
 
 - job:
     name: neutron-tempest-plugin-designate-scenario-rocky