MINOR: Remove unused field in `ListenerConnectionQuota`

Reviewers: Jason Gustafson <jason@confluent.io>

core/src/main/scala/kafka/network/SocketServer.scala

@@ -1228,7 +1228,7 @@ class ConnectionQuotas(config: KafkaConfig, time: Time) extends Logging {
 
   class ListenerConnectionQuota(lock: Object, listener: ListenerName) extends ListenerReconfigurable {
     @volatile private var _maxConnections = Int.MaxValue
-    private val listenerPropName = s"${listener.configPrefix}${KafkaConfig.MaxConnectionsProp}"
+
     def maxConnections: Int = _maxConnections
 
     override def listenerName(): ListenerName = listener