Change lib/neutron-legacy to not enable linuxbridge for DVR

DVR isn't supported by the Linux Bridge agent, but the
mechanism driver is enabled by default, so Neutron attempts
port-bindings for it, generating ERRORS in the neutron-server
log in the check and gate jobs.  Just remove it in the DVR case.

Change-Id: Ic50e12e5fecf366a182c141b5c99649e653254cb
Closes-bug: #1716782
diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index f9e0bd6..0e326d6 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -168,7 +168,7 @@
 #
 Q_DVR_MODE=${Q_DVR_MODE:-legacy}
 if [[ "$Q_DVR_MODE" != "legacy" ]]; then
-    Q_ML2_PLUGIN_MECHANISM_DRIVERS=openvswitch,linuxbridge,l2population
+    Q_ML2_PLUGIN_MECHANISM_DRIVERS=openvswitch,l2population
 fi
 
 # Provider Network Configurations