Exports of Pivot Table contains pivot-grouping when question is unsaved

[flamber]

Describe the bug
When using the visualization Pivot Table and the question is not saved (in "dirty" state), then exports include the column pivot-grouping.

To Reproduce

1. Custom question > Sample Dataset > Products - summarize Count by Category and CreatedAt:Year:
   
2. Change visualization to Pivot Table
   
3. Export as CSV (XLSX or JSON) - the download includes the column pivot-grouping

Category,Created At,pivot-grouping,Count
Doohickey,2016-01-01,0,13
...

4. Saving the question and exporting again, now it only includes the columns as expected:

Category,Created At,Count
Doohickey,2016-01-01,13
...

Expected behavior
Never expose system data like pivot-grouping to users.

Information about your Metabase Installation:
Tested 44ce694 (almost 0.38.1)

Additional context
This will likely get fixed when #15008 is addressed.
Found during testing of #15019