Rely on entity building instead of programmatically saving user input

Problem/Motivation

The multi-step form progress is currently programmatically saved in the form state using a validation handler. However, Drupal Core supplies this out of the box for entity forms.

Proposed resolution

Rely on entity building instead of programmatically saving user input

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.