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