MINOR: Remove duplicate `subscribe` call in ConsumerPerformance (#5828)

In the `consume` method, the consumer subscribes the topic, so no need to do
the same thing before the method call. Also include minor clean-up in `consume`.

Reviewers: Ismael Juma <ismael@juma.me.uk>

core/src/main/scala/kafka/tools/ConsumerPerformance.scala

@@ -27,7 +27,7 @@ import org.apache.kafka.common.serialization.ByteArrayDeserializer
 import org.apache.kafka.common.utils.Utils
 import org.apache.kafka.common.{Metric, MetricName, TopicPartition}
 import kafka.utils.{CommandLineUtils, ToolsUtils}
-import java.util.{Collections, Properties, Random}
+import java.util.{Properties, Random}
 import java.text.SimpleDateFormat
 import java.time.Duration
 
@@ -54,7 +54,6 @@ object ConsumerPerformance extends LazyLogging {
 
     var startMs, endMs = 0L
     val consumer = new KafkaConsumer[Array[Byte], Array[Byte]](config.props)
-    consumer.subscribe(Collections.singletonList(config.topic))
     startMs = System.currentTimeMillis
     consume(consumer, List(config.topic), config.numMessages, config.recordFetchTimeoutMs, config, totalMessagesRead, totalBytesRead, joinGroupTimeInMs, startMs)
     endMs = System.currentTimeMillis
@@ -121,10 +120,9 @@ object ConsumerPerformance extends LazyLogging {
       }})
 
     // Now start the benchmark
-    val startMs = System.currentTimeMillis
-    var lastReportTime: Long = startMs
-    var lastConsumedTime = System.currentTimeMillis
-    var currentTimeMillis = lastConsumedTime
+    var currentTimeMillis = System.currentTimeMillis
+    var lastReportTime: Long = currentTimeMillis
+    var lastConsumedTime = currentTimeMillis
 
     while (messagesRead < count && currentTimeMillis - lastConsumedTime <= timeout) {
       val records = consumer.poll(Duration.ofMillis(100)).asScala