Remove kubeconfig API endpoint

[twz123]

Description

Since the refactoring to use Calico host containers on Windows, there's no need for this endpoint anymore. Remove it along with its associated bootstrap token usage. Also, replace the API authorization based on the k0s "role" with an authorization based directly on the bootstrap token usages, since only the "controller" role was left.

See:

• Windows calico host containers #3209

Type of change

• 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 not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings