Fix BlockBasedTableTest.BlockCacheLeak valgrind failure

Summary:
I added this line in my previous patch D48999 (which is incorrect)
We should not release the iterator since releasing it will evict the blocks from cache

Test Plan:
Run the test under valgrind
make check

Reviewers: rven, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D52161

table/table_test.cc

@@ -1807,7 +1807,6 @@ TEST_F(BlockBasedTableTest, BlockCacheLeak) {
     iter->Next();
   }
   ASSERT_OK(iter->status());
-  iter.release();
 
   const ImmutableCFOptions ioptions1(opt);
   ASSERT_OK(c.Reopen(ioptions1));