Remove rpc_backend setting on newton and ocata.

This setting was deprecated for removal in newton, replaced by
transport_url.  These configurations are also setting transport_url, so
the option is ignored.

This removes a warning issued on service startup:

```
WARNING oslo_config.cfg [-] Option "rpc_backend" from group "DEFAULT" is deprecated for removal.  Its value may be silently ignored in the future.
```
diff --git a/neutron/files/newton/neutron-generic.conf.Debian b/neutron/files/newton/neutron-generic.conf.Debian
index 2f4b89e..e182993 100644
--- a/neutron/files/newton/neutron-generic.conf.Debian
+++ b/neutron/files/newton/neutron-generic.conf.Debian
@@ -537,7 +537,6 @@
 # The messaging driver to use, defaults to rabbit. Other drivers include amqp
 # and zmq. (string value)
 #rpc_backend = rabbit
-rpc_backend = rabbit
 
 # The default exchange under which topics are scoped. May be overridden by an
 # exchange name specified in the transport_url option. (string value)
diff --git a/neutron/files/newton/neutron-server.conf.Debian b/neutron/files/newton/neutron-server.conf.Debian
index 1ae886b..7d13a3f 100644
--- a/neutron/files/newton/neutron-server.conf.Debian
+++ b/neutron/files/newton/neutron-server.conf.Debian
@@ -547,12 +547,6 @@
 # not set, we fall back to the rpc_backend option and driver specific
 # configuration. (string value)
 #transport_url = <None>
-
-# The messaging driver to use, defaults to rabbit. Other drivers include amqp
-# and zmq. (string value)
-#rpc_backend = rabbit
-rpc_backend = rabbit
-
 {%- if server.message_queue.members is defined %}
 transport_url = rabbit://{% for member in server.message_queue.members -%}
                              {{ server.message_queue.user }}:{{ server.message_queue.password }}@{{ member.host }}:{{ member.get('port', 5672) }}
@@ -563,6 +557,11 @@
 transport_url = rabbit://{{ server.message_queue.user }}:{{ server.message_queue.password }}@{{ server.message_queue.host }}:{{ server.message_queue.port }}/{{ server.message_queue.virtual_host }}
 {%- endif %}
 
+# The messaging driver to use, defaults to rabbit. Other drivers include amqp
+# and zmq. (string value)
+#rpc_backend = rabbit
+
+
 # The default exchange under which topics are scoped. May be overridden by an
 # exchange name specified in the transport_url option. (string value)
 #control_exchange = neutron
diff --git a/neutron/files/ocata/neutron-generic.conf.Debian b/neutron/files/ocata/neutron-generic.conf.Debian
index 6275974..9749659 100644
--- a/neutron/files/ocata/neutron-generic.conf.Debian
+++ b/neutron/files/ocata/neutron-generic.conf.Debian
@@ -605,7 +605,6 @@
 # Its value may be silently ignored in the future.
 # Reason: Replaced by [DEFAULT]/transport_url
 #rpc_backend = rabbit
-rpc_backend = rabbit
 
 # The default exchange under which topics are scoped. May be overridden by an
 # exchange name specified in the transport_url option. (string value)
diff --git a/neutron/files/ocata/neutron-server.conf.Debian b/neutron/files/ocata/neutron-server.conf.Debian
index 0047550..db88080 100644
--- a/neutron/files/ocata/neutron-server.conf.Debian
+++ b/neutron/files/ocata/neutron-server.conf.Debian
@@ -624,7 +624,6 @@
 # Its value may be silently ignored in the future.
 # Reason: Replaced by [DEFAULT]/transport_url
 #rpc_backend = rabbit
-rpc_backend = rabbit
 
 # The default exchange under which topics are scoped. May be overridden by an
 # exchange name specified in the transport_url option. (string value)