fix(core): prevent validation error in version/publish with workspace: prefix

[fahslaj]

Description

Prevents the validation error when the Package Graph is constructed without all of the dependencies listed by all packages.

This PR also removes an unused function from the package-graph unit tests.

Motivation and Context

During the changelog update of version and publish, the package graph is constructed with only the changed packages, so it can't make assumptions based on the existence of other packages in the workspace. In short, there is not enough information to properly validate what the validation error was asserting in all use cases of PackageGraph.

#3317

How Has This Been Tested?

This change reverts a previously-added validation error. Tests checking for that validation error have been removed and all other unit/integration/e2e tests have passed. Lerna version and publish have also been run manually to verify that this change solves the issue above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (change that has absolutely no effect on users)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.