Remove io.Writer from fs.File

It was only used in a single test, which now uses plain *os.File instead.

internal/fs/helpers.go

@@ -1,10 +1,6 @@
 package fs
 
-import (
-	"io/ioutil"
-	"os"
-	"testing"
-)
+import "os"
 
 // IsRegularFile returns true if fi belongs to a normal file. If fi is nil,
 // false is returned.
@@ -15,22 +11,3 @@ func IsRegularFile(fi os.FileInfo) bool {
 
 	return fi.Mode()&(os.ModeType|os.ModeCharDevice) == 0
 }
-
-// TestTempFile returns a new temporary file, which is removed when cleanup()
-// is called.
-func TestTempFile(t testing.TB, prefix string) (File, func()) {
-	f, err := ioutil.TempFile("", prefix)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	cleanup := func() {
-		_ = f.Close()
-		err = Remove(f.Name())
-		if err != nil {
-			t.Fatal(err)
-		}
-	}
-
-	return f, cleanup
-}