Stats: Update "billable_monthly_views" by excluding the purchased plan quantity

[dognose24]

Related to https://github.com/Automattic/red-team/issues/15

Proposed Changes

• Use billable_monthly_views by excluding the purchased plan quantity from the API end to determine the purchasable Stats commercial tiers.

Why are these changes being made?

• Use the calculated count from the API rather than do it locally.

Testing Instructions

• Apply the Diff D152811-code and follow its testing instructions.
• Prepare a self-hosted site with only a Jetpack Complete plan.
• Modify the variable $billable_monthly_views from the API end code to 109999.
• Spin this change up with the Calypso Live link.
• Navigate to the Stats commercial purchase page: /stats/purchase/{site-slug}?productType=commercial.
• Ensure the first purchase tier is 10K.

• Modify the variable $billable_monthly_views from the API end code to 110000.
• Reload the purchase page.
• Ensure the first purchase tier is 100K.

Pre-merge Checklist

• [ ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• [ ] Have you written new tests for your changes?
• [ ] Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• [ ] Have you checked for TypeScript, React or other console errors?
• [ ] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [ ] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [ ] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?