Do not install libvirt python bindings from pip

Since [1] was merged, devstack no longer installs libvirt python
bindings from pip but use distro packages. This change removes
the logic to install the python bindings using pip to follow that
change in devstack, otherwise installation fails because of conflict
between packages available in pip and one installed by distro packages.

[1] https://review.opendev.org/c/openstack/devstack/+/798514

Conflicts:
	devstack/plugin.sh

Resolved conflicts caused by f57a51e43c
which is not backported to stable branches.

Closes-Bug: #1939990
Related-Bug: #1933096
Change-Id: I369a95cf30e91738d10ad2e5a3d5f6988bdd76cb
(cherry picked from commit 9366bc8f18)

devstack/plugin.sh

@@ -97,9 +97,8 @@ function _ceilometer_prepare_coordination {
 function _ceilometer_prepare_virt_drivers {
     # Only install virt drivers if we're running nova compute
     if is_service_enabled n-cpu ; then
-        if [[ "$VIRT_DRIVER" = 'libvirt' ]]; then
-            pip_install_gr libvirt-python
-        fi
+        # NOTE(tkajinam): pythonN-libvirt is installed using distro
+        # packages in devstack
 
         if [[ "$VIRT_DRIVER" = 'vsphere' ]]; then
             pip_install_gr oslo.vmware