Site Migration: Remove migration instruction i2 flag code

[markbiek]

Related to #90502

Proposed Changes

We had been using the migration-flow/remove-processing-step flag to gate the updates to the migration flow where we skip the Processing and bundleTransfer steps.

We've determined that this change is helping get users through the flow so we're removing the flag.

Why are these changes being made?

In #90502, we're going to update the /setup/migration-signup flow to behave in the same way as the current /setup/site-migration flow. We want to go directly to the Migration Instructions step and have the Atomic transfer and plugin install happen behind the scenes.

As a first step towards making that change, we want to remove all of the migration-flow/remove-processing-step flag-specific code.

Testing Instructions

• Tests should pass

yarn test-client client/landing/stepper/declarative-flow/test/site-migration-flow.tsx

• Using the calypso.live url or running this locally, visit /start.
• On the Goals step, select the "Import existing content or website" goal.
• Enter a valid WordPress site on the "Import From" step.
• Chose a Free Trial or upgrade to the Creator plan on the Upgrade Plan step.
• Verify that you're then taken to the new Migration Instructions step.
• Verify that all of the background actions on the instructions step complete successfully.

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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-107497

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-107497&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-107497&env=a8c-for-agencies

[matticbot]

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

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

• blaze-dashboard
• odyssey-stats
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/site-migration-remove-flag on your sandbox.

[matticbot]

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

Sections (~207 bytes removed 📉 [gzipped])

name                 parsed_size           gzip_size
site-migration-flow       -707 B  (-1.7%)     -207 B  (-2.9%)

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.

[sixhours]

This is working, just looks like it needs a rebase.