Social Auth: Remove Google API migration feature flag and obsolete code

[zaguiini]

Proposed Changes

Fixes #64791. Fixes #64538.

After a few weeks of having the migration/sign-in-with-google flag enabled in production, we did not notice any substantial reduction in conversions. This is backed by data: 2d132-pb.

Testing Instructions

The same as #64957, except for the need to activate the migration/sign-in-with-google flag (which is now the default).

Pre-merge Checklist

• 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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-39162

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-39162&env=jetpack

[matticbot]

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

App Entrypoints (~583 bytes removed 📉 [gzipped])

name         parsed_size           gzip_size
entry-login      -1928 B  (-0.2%)     -583 B  (-0.2%)

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

Sections (~1153 bytes removed 📉 [gzipped])

name             parsed_size           gzip_size
jetpack-connect      -2039 B  (-0.2%)     -633 B  (-0.2%)
security             -1929 B  (-0.4%)     -520 B  (-0.4%)
accept-invite        -1916 B  (-0.4%)     -574 B  (-0.6%)

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

Async-loaded Components (~643 bytes removed 📉 [gzipped])

name                          parsed_size           gzip_size
async-load-design-blocks          -1928 B  (-0.1%)     -593 B  (-0.1%)
async-load-signup-steps-user      -1916 B  (-1.0%)     -574 B  (-1.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

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.

[danielbachhuber]

👍 Works well in my testing!

I was able to:

• Successfully log in with my Google account.
• Successfully sign up with my Google account.
• Successfully connect and disconnect my Google account.