Billing: Cancelling plans is quite slow

#### Steps to reproduce

See 3232-gh-jpop-issues

#### What I expected

The cancellation of a plan to take much less time.

#### What happened instead

Plan cancellation takes enough time that it seems to have failed.

#### Browser / OS version

N/A

#### Screenshot / Video

See 3232-gh-jpop-issues

#### Context / Source

This issue was reported by @annezazu quite some time ago. From my debugging it appears that the issue is that the endpoint returns a list of all of the user's purchases after cancelling a purchase. For Automatticans that test, or users with many sites, this going to be a slow process.

In my opinion, the fix here is to stop returning the list of purchases after cancelling a single purchase and to then update clients to delete the single purchase after a successful API call.

Start by tracing the plan-cancellation API endpoint and the client code that handles its response. Confirm the endpoint no longer returns the full purchase list and that clients remove the cancelled purchase after a successful API call; verify cancellation no longer appears to stall for users with many sites or purchases.

Written by the indexing model from the issue text.