[Custom Fields] Store changes cont: remove subscriptions data from WCProductModel

[hichamboushaba]

While we moved most metadata to its own table, we still store subscriptions data inside the WCProductModel table, we need to extract it as well.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.