Billing: Remove setting CartStore site id in ManagePurchase

[sirbrillig]

Changes proposed in this Pull Request

The ManagePurchase component caches the currently selected site ID in the CartStore. I'm not 100% sure why this is the case, but presumably it was used when later adding products to the cart (renewals, for example). In any case, as most uses of the CartStore have been removed (see #24019), I suspect this use no longer has any value and remove it here.

Testing instructions

Verify that viewing a purchase works as expected.

[matticbot]

Test live: https://calypso.live/?branch=remove/cart-store-use-manage-purchase

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~70 bytes added 📈 [gzipped])

name                   parsed_size           gzip_size
entry-main                  +105 B  (+0.0%)      +30 B  (+0.0%)
entry-login                 +105 B  (+0.0%)      +32 B  (+0.0%)
entry-gutenboarding         +105 B  (+0.0%)      +28 B  (+0.0%)
entry-domains-landing       +105 B  (+0.0%)      +37 B  (+0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Sections (~57 bytes removed 📉 [gzipped])

name            parsed_size           gzip_size
site-purchases       -230 B  (-0.0%)      -57 B  (-0.0%)
purchases            -230 B  (-0.0%)      -57 B  (-0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[nbloomf]

Looks good. All the doo-dads on the /me/purchases management page seem to work as expected.