feat(starter-content): e2e improvements; removal CLI command

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Small improvements to how starter content is handled.

How to test the changes in this Pull Request:

1. Generate starter content with wp newspack setup
2. Remove it with wp newspack remove-starter-content – observe it's removed
3. Add NEWSPACK_IS_E2E environment variable and generate starter content again
4. Observe all posts have the same image and content, and the titles are prefixed with the post index

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?