chore: remove Virtual Machine ScaleSets support.

[jadarsie]

Reason for Change:

AKS Engine on Azure Stack Hub does not support VMSS pools.

This PR removes the VMSS ARM client and consumer code.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• [ ] No
• [ ] Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• [ ] No
• [ ] Yes

Requirements:

• [ ] uses conventional commit messages
• [ ] includes documentation
• [ ] adds unit tests
• [ ] tested upgrade from previous version

Notes: