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/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"`