MINOR: Removed unused imports in a few tests (#4938)

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

core/src/test/scala/unit/kafka/network/SocketServerTest.scala

@@ -20,7 +20,7 @@ package kafka.network
 import java.io._
 import java.net._
 import java.nio.ByteBuffer
-import java.util.{HashMap, Properties, Random}
+import java.util.{HashMap, Random}
 import java.nio.channels.SocketChannel
 import javax.net.ssl._
 

core/src/test/scala/unit/kafka/server/DelegationTokenRequestsOnPlainTextTest.scala

@@ -16,7 +16,6 @@
   */
 package kafka.server
 
-import java.nio.ByteBuffer
 import java.util
 
 import kafka.utils.TestUtils

core/src/test/scala/unit/kafka/server/DelegationTokenRequestsWithDisableTokenFeatureTest.scala

@@ -16,7 +16,6 @@
   */
 package kafka.server
 
-import java.nio.ByteBuffer
 import java.util
 
 import kafka.api.{KafkaSasl, SaslSetup}