Remove references to the neutron:server:plugin parameter

This parameter is present in service- and system-level metadata
even though it became oblosete somewhere around adoption of Kilo
release of OpenStack when it was replased by the
neutron:server:backend:engine param. Its presense currently only
clutters metadata and confuses users.

The same cleunup is also performed on the neutron formula level:
https://gerrit.mcp.mirantis.net/10753

Change-Id: Ib29e061cf5174aa93d1ae1f1f2b2483408b6d6e2
diff --git a/neutron/control/openvswitch/cluster.yml b/neutron/control/openvswitch/cluster.yml
index c840e9a..c86db48 100644
--- a/neutron/control/openvswitch/cluster.yml
+++ b/neutron/control/openvswitch/cluster.yml
@@ -13,7 +13,6 @@
     neutron_enable_vlan_aware_vms: False
   neutron:
     server:
-      plugin: ml2
       global_physnet_mtu: ${_param:neutron_global_physnet_mtu}
       l3_ha: ${_param:neutron_l3_ha}
       dvr: ${_param:neutron_control_dvr}
diff --git a/neutron/control/openvswitch/single.yml b/neutron/control/openvswitch/single.yml
index 1b830f6..27cee80 100644
--- a/neutron/control/openvswitch/single.yml
+++ b/neutron/control/openvswitch/single.yml
@@ -11,7 +11,6 @@
     neutron_enable_vlan_aware_vms: False
   neutron:
     server:
-      plugin: ml2
       global_physnet_mtu: ${_param:neutron_global_physnet_mtu}
       l3_ha: ${_param:neutron_l3_ha}
       dvr: ${_param:neutron_control_dvr}