Remove default logic for Flux resource fetch

Proposal

Currently, we have a default logic for fetching the Flux resource on the Environments page - we search for either Kustomization or helmRelease resource with a name equal to the environment's name. As we discussed during the design pairing meeting, we want to simplify the logic and only fetch the resource that the user has specified on the environments settings page.

This way we are avoiding sending multiple requests - both for the Kustomization and HelmRelease with the same name. Only one request will be needed instead.