Removing unused image tags

After upversioning openstack-helm and openstack-helm-infra, update
versions of helm charts introduced new docker images that are not used
by our stx-openstack distro but would still be downloaded during an
application apply if the helm values are not set to null.

This change removes unnecessary neutron-ovn and
libvirtd-export images.

Closes-Bug: 2043037

Test Plan:
PASS - Build stx-openstack-helm-fluxcd
PASS - Build stx-openstack application tarball
PASS - Upload/Apply/Remove stx-openstack application

Change-Id: I36bbde03785ad11b0cf97a9cf3e357ebd8084175
Signed-off-by: Thales Elero Cervi <thaleselero.cervi@windriver.com>

stx-openstack-helm-fluxcd/stx-openstack-helm-fluxcd/manifests/libvirt/libvirt-static-overrides.yaml

@@ -39,4 +39,5 @@ images:
     ceph_config_helper: docker.io/openstackhelm/ceph-config-helper:ubuntu_bionic-20201223
     image_repo_sync: null
     libvirt: docker.io/starlingx/stx-libvirt:master-debian-stable-latest
+    libvirt_exporter: null
 ...

stx-openstack-helm-fluxcd/stx-openstack-helm-fluxcd/manifests/neutron/neutron-static-overrides.yaml

@@ -154,6 +154,7 @@ images:
     neutron_metadata: docker.io/starlingx/stx-neutron:master-debian-stable-latest
     neutron_netns_cleanup_cron: docker.io/starlingx/stx-neutron:master-debian-stable-latest
     neutron_openvswitch_agent: docker.io/starlingx/stx-neutron:master-debian-stable-latest
+    neutron_ovn_metadata: null
     neutron_server: docker.io/starlingx/stx-neutron:master-debian-stable-latest
     neutron_sriov_agent: docker.io/starlingx/stx-neutron:master-debian-stable-latest
     neutron_sriov_agent_init: docker.io/starlingx/stx-neutron:master-debian-stable-latest