✨ api/v1alpha4 remove obsolete UserDataSecret field

[chrischdi]

What this PR does / why we need it:

Removes the unused field UserDataSecret in v1alpha4.
It was used in an earlier CAPI version (v1alpha1 or v1alpha2) to reference a secret which contains the user data, but is not anymore.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #857

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• if necessary:
  • includes documentation
  • adds unit tests

/hold

[k8s-ci-robot]

Hi @chrischdi. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[hidekazuna]

/ok-to-test

[chrischdi]

I messed up the conversion, I'll take a look at it :-)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrischdi, sbueringer

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[jichenjc]

/lgtm
/hold cancel