Remove vertical-related features from the onboarding

## Description

Initially discussed in pekYwv-rK-p2, we want to remove vertical-related features from the onboarding now that it's no longer developed. In order to start removing these features, we first need to figure out if they are being used by other teams. The list of features involves:

- The site vertical step in onboarding.
- The site vertical API endpoint.
- Generated designs.
- MC recipe tool.

Update: After discussion in p1681783642584379-slack-C048CUFRGFQ, the scope for the initial step will be just removing generated designs, and image verticalization in the Onboarding Design Picker.

## Acceptance Criteria
- [x] Investigate which vertical-related features to remove.
- [x] Remove generated designs in the Onboarding Design Picker
- [ ] Remove image verticalization in the Onboarding Design Picker

## Dev Tasks
- [x] https://github.com/Automattic/wp-calypso/pull/75999
- [x] D108448-code
- [x] https://github.com/Automattic/wp-calypso/pull/76114
- [x] D108722-code