Remove references to `required` key in props

What does this MR do?

Remove section stating that the required key is required on optional props.

Ideally styleguide rules should be enforced by machines, eslint or prettier. I had no luck finding an eslint-plugin to enforce this rule.

So we need to manually enforce this rule during review.

That's extra work for authors and reviewers, for the benefit of the readability that conformity brings.

We haven't been doing a great job at enforcing this manually: it's easy to find instances that break this rule so let's evaluate if we want to keep this rule.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

- [ ] Changelog entry docs only

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content