[FLINK-23476][build] Remove unused dependency

The akka.version property was recently moved to flink-rpc-akka, but there was still a reference to that property from the unused akka-testkit dependency in the root pom.

pom.xml

@@ -660,13 +660,6 @@ under the License.
 				<version>1.1.1</version>
 			</dependency>
 
-			<dependency>
-				<groupId>com.typesafe.akka</groupId>
-				<artifactId>akka-testkit_${scala.binary.version}</artifactId>
-				<version>${akka.version}</version>
-				<scope>test</scope>
-			</dependency>
-
 			<dependency>
 				<groupId>org.scalatest</groupId>
 				<artifactId>scalatest_${scala.binary.version}</artifactId>
@@ -1198,21 +1191,6 @@ under the License.
 					</plugin>
 				</plugins>
 			</build>
-			<dependencies>
-				<dependency>
-					<!--
-					This is necessary for building the java docs using Java 8. Otherwise the javadoc
-					plugin will fail with "javadoc: error -
-						com.sun.tools.doclets.internal.toolkit.util.DocletAbortException:
-							com.sun.tools.javac.code.Symbol$CompletionFailure:
-								class file for akka.testkit.TestKit not found"
-					-->
-					<groupId>com.typesafe.akka</groupId>
-					<artifactId>akka-testkit_${scala.binary.version}</artifactId>
-					<version>${akka.version}</version>
-					<scope>provided</scope>
-				</dependency>
-			</dependencies>
 		</profile>
 
 		<profile>