Remove redundant 'plan' from order summary details

This addresses customers-gitlab-com#7261 (closed).

What does this MR do and why?

This MR removes a redundant occurrence of the word 'plan' in the checkout page. Please refer to the screenshots below.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to the checkout page (e.g. http://localhost:3000/-/subscriptions/new?namespace_id=189&plan_id=2c92c0f876e0f4cc0176e176a08f1b70)
2. Verify the word 'plan' is not repeated

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.