refactor!: stop removing old .pnp.js files when migrating

[paul-soporan]

What's the problem this PR addresses?

We still remove the old .pnp.js files when migrating, but our telemetry currently shows that only 5% of all modern users still use Yarn 2, so it doesn't make sense for us to still keep this migration feature one more major when they can just migrate one major at a time.

How did you fix it?

Removed the code.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.