capz: remove WINDOWS and TEST_WINDOWS env vars

[jackfrancis]

When not requiring Windows infra for tests in CAPZ, you don't have to explicitly set any configuration. This simplifies the jobs so that explicit usages like WINDOWS=false or TEST_WINDOWS=false are removed.

[jackfrancis]

/assign @mborsz @wojtek-t

(just a housekeeping PR but it will help keep our sanity managing all the CAPZ tests across many projects)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jackfrancis, mboersma, nojnhuh Once this PR has been reviewed and has the lgtm label, please ask for approval from wojtek-t. For more information see the Code Review Process.

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[mboersma]

/assign @mm4tt

[jackfrancis]

gentle ping @mborsz @wojtek-t @mm4tt