Remove redundant join from unit test (#2807)

We already join on this future on line 185. This second join is
redundant and will result in an exception being thrown.

Author: Bennett-Lynch

services-custom/s3-transfer-manager/src/test/java/software/amazon/awssdk/transfer/s3/internal/S3TransferManagerListenerTest.java

@@ -204,7 +204,6 @@ public void upload_failure_shouldInvokeListener() throws Exception {
         assertThat(ctx2.progressSnapshot().bytesTransferred()).isZero();
         assertThat(ctx2.exception()).isInstanceOf(NoSuchFileException.class);
 
-        upload.completionFuture().join();
         verifyNoMoreInteractions(listener);
     }