Don't advertise the BungeeCord plugin messaging channel if it is disabled.

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -65,11 +65,14 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
   @Override
   public void activated() {
     serverConn.getServer().addPlayer(serverConn.getPlayer());
+
+    if (server.getConfiguration().isBungeePluginChannelEnabled()) {
       MinecraftConnection serverMc = serverConn.ensureConnected();
       serverMc.write(PluginMessageUtil.constructChannelsPacket(serverMc.getProtocolVersion(),
           ImmutableList.of(getBungeeCordChannel(serverMc.getProtocolVersion()))
       ));
     }
+  }
 
   @Override
   public boolean beforeHandle() {