chore: remove marian's duplicate installation check

marian.Installed and marian.Files were only used by the package's own
test, while the app has its own check in translation.go - with a
different rule (pinned size there, size > 0 here), so the two
definitions of 'installed' could disagree. The app's check is the
canonical one (it owns the pins); Load already fails on an incomplete
model, which the test keeps asserting.

internal/marian/download.go

@@ -21,16 +21,6 @@ type RemoteFile struct {
 	SHA256 string
 }
 
-// Installed reports whether dir holds every file of a model.
-func Installed(dir string) bool {
-	for _, f := range Files {
-		if st, err := os.Stat(filepath.Join(dir, f)); err != nil || st.Size() == 0 {
-			return false
-		}
-	}
-	return true
-}
-
 // Download fetches files into dir. A file already there with the right
 // size and digest is skipped. A file is downloaded into a ".partial" file
 // that is renamed only once its digest checks out, so an interrupted

internal/marian/marian_test.go

@@ -251,9 +251,6 @@ func tinyModel(t *testing.T) string {
 
 func TestTinyModel(t *testing.T) {
 	dir := tinyModel(t)
-	if !Installed(dir) {
-		t.Fatal("tiny model not installed")
-	}
 	m, err := Load(context.Background(), dir)
 	if err != nil {
 		t.Fatal(err)
@@ -281,9 +278,6 @@ func TestTinyModel(t *testing.T) {
 	}
 
 	os.Remove(filepath.Join(dir, "vocab.json"))
-	if Installed(dir) {
-		t.Error("incomplete model reported installed")
-	}
 	if _, err := Load(context.Background(), dir); err == nil {
 		t.Error("incomplete model loaded")
 	}

internal/marian/model.go

@@ -26,9 +26,6 @@ import (
 	"path/filepath"
 )
 
-// Files are the files a model directory must contain.
-var Files = []string{"config.json", "model.safetensors", "source.spm", "target.spm", "vocab.json"}
-
 type config struct {
 	DModel          int    `json:"d_model"`
 	EncoderLayers   int    `json:"encoder_layers"`