Fix: remove unused GrpcCleanupRule import from GrpcXdsTransportFactoryTest to fix checkstyle

Author: AgraVator

xds/src/test/java/io/grpc/xds/GrpcXdsTransportFactoryTest.java

@@ -45,7 +45,6 @@
 import io.grpc.Server;
 import io.grpc.Status;
 import io.grpc.stub.StreamObserver;
-import io.grpc.testing.GrpcCleanupRule;
 import io.grpc.testing.TestMethodDescriptors;
 import io.grpc.xds.client.Bootstrapper;
 import io.grpc.xds.client.XdsTransportFactory;