Remove support for storing the data model without using multipart

[olemartinorg]

Multipart saving was implemented in #899 as an alternative to sending the component ID that triggered saving to the backend. After Altinn/app-lib-dotnet#187 is implemented, and if it's implemented in a backend version we can require in app-frontend, we should remove support for the old form data saving methods.

[olemartinorg]

Closed in #1712