spare sylva-units HelmRelease from force-reconcile-helmreleases Kyverno policy

During a discussion today we concluded that it would probably be better if the force-reconcile-helmreleases Kyverno policy (that triggers forceful reconciliation of Flux HelmReleases) would not be triggering on sylva-units HelmRelease. It does not indeed seems useful and not doing that would make our system more robust (by not depending on Kyverno webhook availability to update sylva-units HelmRelease).

/cc @feleouet