Remove FF and tests for npm6 deprecation and unsupported error

[amazimbe]

What are you trying to accomplish?

NPM 6 was deprecated last December and it's no longer supported. However, that change was released under two feature flags that are now no longer needed. This PR removes the feature flags. Also note that the code in npm_and_yarn that detects the user version is controlled by 3 other feature flags so the tests have been updated accordingly under these new conditions.

Improve readability and maintainability by removing unnecessary feature flags and conditional statements.

How will you know you've accomplished your goal?

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[markhallen]

LGTM

[robaiken]

The submodule NuGet.Client has been deleted, this change needs to be reverted

[amazimbe]

The submodule NuGet.Client has been deleted, this change needs to be reverted

Well spotted! I noticed it too and I'm removing it.