🐛 Remove IdentityRef.Kind

[EmilienM]

What this PR does / why we need it:

Delete the identityRef.Kind field.
It was used to specify the kind of the identity provider to use but was actually ignored.

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 #1882

TODOs:

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

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[EmilienM]

/cc stephenfin

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: EmilienM, mdbooth

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• OWNERS [mdbooth]

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[EmilienM]

/cc lentzi90

[lentzi90]

/lgtm
Holding if you want to fix that nit
/hold

[EmilienM]

/hold cancel
I think it's ready.

[mdbooth]

Restoring @lentzi90's lgtm as there were no changes other than the requested nit fix.

/lgtm