block: qcow: tests: Update combo_write_read for cluster leak fixes

Freed clusters correctly have refcount=0. Remove the assertion that
expected no clusters with zero refcount, as it was validating the
buggy behavior.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/mod.rs

@@ -2882,8 +2882,6 @@ mod unit_tests {
                     assert_eq!(orig, read);
                 }
             }
-
-            assert_eq!(qcow_file.first_zero_refcount().unwrap(), None);
         });
     }