sylva-units-operator should not have strong dependency on externa-secret-operator CRDs

While deleting the failed SUR created in #32 (closed)

I faced the following message:

      message: 'Failed to reconcile sylvaUnitsRelease: failed to retrieving ExternalSecret
        /sylva-units for deletion: no matches for kind "ExternalSecret" in version
        "external-secrets.io/v1beta1"'

reconcileDelete shoud succed to delete sylva-units-release in that case.

Even if we'll probably need eso in most case in the future, it would be cleaner to avoid having hard dependency on eso if there are no references to external secrets.