Hi,
There is no global setting, and as it's a system table you can't change it from the admin interface:
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Safest option is as Dmitry suggested to use a Global Events Handler. This way you don't adjust any Kentico core objects.
This however never allows the setting to be set to true.
What you could do, but potentially gets overwritten on a Hotfix or upgrades is change the setting anyway.
"easiest/safest" would be to do it directly in the DB:
The fields config are stored in XML in the 'ClassFormDefinition'.
Note: only do this when just instructing the editor isn't sufficient, and you understand the impact of what you're doing in the DB. Always create a backup, and document this change for future reference when doing upgrades or such.
result after update:
Query I used:
UPDATE cms_class
SET ClassFormDefinition = REPLACE(ClassFormDefinition, '<defaultvalue>ByProduct</defaultvalue><fieldcaption>Track inventory</fieldcaption>', '<defaultvalue>Disabled</defaultvalue><fieldcaption>Track inventory</fieldcaption>')
WHERE ClassName = 'ecommerce.SKU'
NOTE AGAIN: create DB BACKUP FIRST