[neutron] don't enable l2population by default

the l2population mechanism is only necessary when using vxlan tenant
networks, which is not currently supported in chi-in-a-box. It is a no-op
for baremetal nodes with flat or vlan networks, and logs noisy warnings.

It was previously introduced here for chi@edge v1, which used vxlan networks
with Zun, but that was deprecated in v2.

NOTE: this will need to be revisited for KVM support, if vxlan networks are used

kolla/node_custom_config/neutron/ml2_conf.ini

@@ -1,5 +1,5 @@
 [ml2]
-mechanism_drivers = {% if enable_neutron_wireguard | bool %}wireguard,{% endif %}openvswitch,genericswitch,{% if enable_ironic_neutron_agent | bool %}baremetal,{% endif %}l2population
+mechanism_drivers = {% if enable_neutron_wireguard | bool %}wireguard,{% endif %}openvswitch,genericswitch,{% if enable_ironic_neutron_agent | bool %}baremetal{% endif %}
 
 [ml2_type_flat]
 # Allow creating flat networks against any physical provider network