Fixed flaky unpersist test in DistributedSuite.

core/src/test/scala/spark/DistributedSuite.scala

@@ -276,7 +276,6 @@ class DistributedSuite extends FunSuite with ShouldMatchers with BeforeAndAfter
           // is racing this thread to remove entries from the driver.
       }
     }
-    assert(sc.getRDDStorageInfo.isEmpty === true)
   }
 }