Fix sylva-units delete hook

Francois Eleouetrequested to merge
fix-delete-hook into main

What does this MR do and why?

Use specific field-manager in delete-hook

Otherwise kustomize controller could unsuspend HelmReleases during reconciliations of their controlling Kustomizations

See explanations in related issue: #1895 (comment 2231968508)

Related reference(s)

Closes: #1887 (closed) Closes: #1895 (closed)

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