oci/client: drop custom platfrom from default client options

In OCI terms having platform `flux/flux/v2` implies that the image
is runnable on some OS called "flux" on architecture called "flux".
In reality there is no such platform and there is no good reason
for Flux to pretend. OCI 1.1 (albeit still and RC at the moment),
doesn't mandate artifact to define a platform at all.

When client is constructed with a platform selector, many functions
will apply the selector, which is often undesirable.

The `flux/flux/v2` platform was only defined in client options, so
it wasn't set on any images and is safe to drop; source-controller
doesn't use `DefaultOptions`, so there is no concern.

Another inconvenient behaviour of GGCR is that some functions will pick
just the very first element that matches the platform and ignore any
additional elements, e.g. elements with same platform, but different
media type. That behaviour prevents searching for artefacts by media
type.

Signed-off-by: Ilya Dmitrichenko <errordeveloper@gmail.com>

oci/client/client.go

@@ -20,7 +20,6 @@ import (
 	"context"
 
 	"github.com/google/go-containerregistry/pkg/crane"
-	gcrv1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 
 	"github.com/fluxcd/pkg/oci"
@@ -41,16 +40,9 @@ func NewClient(opts []crane.Option) *Client {
 	return &Client{options: options}
 }
 
-// DefaultOptions returns an array containing crane.WithPlatform
-// to set the platform to flux.
+// DefaultOptions returns an empty list of client options.
 func DefaultOptions() []crane.Option {
-	return []crane.Option{
-		crane.WithPlatform(&gcrv1.Platform{
-			Architecture: "flux",
-			OS:           "flux",
-			OSVersion:    "v2",
-		}),
-	}
+	return []crane.Option{}
 }
 
 // GetOptions returns the list of crane.Option used by this Client.