Clearing the ordering of a table does not clear the `ordering` key from the UserConfig data

[jeremystretch]

NetBox version

v3.5.3

Python version

3.8

Steps to Reproduce

1. Open the sites list and order the entries by name.
2. In a shell, retrieve the UserConfig instance for your user and inspect the entry for tables.SiteTable.ordering:

>>> uc=UserConfig.objects.get(user__username='admin')
>>> uc.data['tables']['SiteTable']['ordering']
['name']

3. In the UI, clear the applied ordering by clicking the X in the column header.
4. Refresh and re-inspect the UserConfig data:

>>> uc.refresh_from_db()
>>> uc.data['tables']['SiteTable']['ordering']
['']

Expected Behavior

The ordering key should be removed from the data, as there is no longer any preference stored.

Observed Behavior

ordering is set to a list containing an empty string. This does not effect any breaking behavior AFAICT, however it should be cleaned up as it can pose complications.