Remove "login/social-first" feature flag

[tellthemachines]

Follow-up from #102811.

The "login/social-first" flag has been true in production env for 2 years now.

The only place it's still set to false is the desktop config, but desktop app logins are now routed through the browser so the flag is no longer relevant in that context.

This PR only deals with removing the flag. There's some confusing logic around the use of the isSocialFirst variable which can be dealt with afterwards. That variable is also defined in here without reference to the feature flag, and here it's given a default value of false.

Testing Instructions

• Check that /log-in/ related paths are unchanged;
• For the desktop app, follow instructions to start it up locally and check that its login screen looks identical to the production version:

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 UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• 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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-f3df33d86d9036ed44ce2df90cf376009c516538

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-f3df33d86d9036ed44ce2df90cf376009c516538&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-f3df33d86d9036ed44ce2df90cf376009c516538&env=a8c-for-agencies

[matticbot]

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

App Entrypoints (~8 bytes removed 📉 [gzipped])

Details

name                    parsed_size           gzip_size
entry-login                   -62 B  (-0.0%)       -8 B  (-0.0%)
entry-subscriptions           -24 B  (-0.0%)       -2 B  (-0.0%)
entry-stepper                 -24 B  (-0.0%)       -2 B  (-0.0%)
entry-reauth-required         -24 B  (-0.0%)       -2 B  (-0.0%)
entry-main                    -24 B  (-0.0%)       -2 B  (-0.0%)
entry-domains-landing         -24 B  (-0.0%)       -2 B  (-0.0%)
entry-dashboard-dotcom        -24 B  (-0.0%)       -2 B  (-0.0%)
entry-dashboard-a4a           -24 B  (-0.0%)       -2 B  (-0.0%)

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

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.

[escapemanuele]

Thanks for cleaning up, it was much needed.
I'll try to see if I can remove what's left of the social-first logic 👍

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• blaze-dashboard
• happy-blocks
• help-center
• notifications
• odyssey-stats
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug remove/social-first-feature-flag on your sandbox.