[receiver/zipkin] Remove unused locking (#3463)

The lock wasn't called from more than one location.

Relates #3041

receiver/zipkinreceiver/trace_receiver.go

@@ -49,9 +49,6 @@ var errNextConsumerRespBody = []byte(`"Internal Server Error"`)
 
 // ZipkinReceiver type is used to handle spans received in the Zipkin format.
 type ZipkinReceiver struct {
- // mu protects the fields of this struct
- mu sync.Mutex
-
  // addr is the address onto which the HTTP server will be bound
  host component.Host
  nextConsumer consumer.Traces
@@ -95,9 +92,6 @@ func (zr *ZipkinReceiver) Start(_ context.Context, host component.Host) error {
  return errors.New("nil host")
  }
 
- zr.mu.Lock()
- defer zr.mu.Unlock()
-
  zr.host = host
  zr.server = zr.config.HTTPServerSettings.ToServer(zr)
  var listener net.Listener