bigquery: reduce test flakiness when testing job cancellation. (#5592)

BigQuery's cancellation mechanism is best-effort notification to
the job server running cancellation.  Depending on a timing factors,
the cancellation can be ignored, arrive after job completion, or
arrive in time to trigger state transition (which will yield an error).

This change removes the expectation that cancelling a job won't
yield an error, which is incorrect for the last case.

Author: shollyman

google-cloud-clients/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/it/ITBigQueryTest.java

@@ -1451,7 +1451,6 @@ public void testCancelJob() throws InterruptedException, TimeoutException {
     Job remoteJob = bigquery.create(JobInfo.of(configuration));
     assertTrue(remoteJob.cancel());
     remoteJob = remoteJob.waitFor();
-    assertNull(remoteJob.getStatus().getError());
   }
 
   @Test