Revert appLogic.allowInSubform requirement

[danielskovli]

Description

The subform concept was initially configured in a way where the app developer had to declare applicationmedadata->dataTypes->applogic.allowInSubform in order for the data model to be usable as a subform.

This functionality is no longer necessary: Slack thread.

Solution

• Storage: Mark ApplicationLogic.AllowInSubform as obsolete
• Frontend: Remove allowInSubform check from SubformValidator and all type+mock references: https://github.com/search?q=repo%3AAltinn%2Fapp-frontend-react%20allowinsubform&type=code
• Docs: Remove reference from app metadata page