cleanup: move legacy from internal/engine to internal (#759)

No functional change.

See https://github.com/ooni/probe/issues/2115

internal/engine/legacy/assetsdir/assetsdir_test.go

@@ -1,40 +0,0 @@
-package assetsdir
-
-import (
-	"errors"
-	"strings"
-	"testing"
-)
-
-func TestCleanupNormalUsage(t *testing.T) {
-	result, err := Cleanup("testdata")
-	if err != nil {
-		t.Fatal(err)
-	}
-	// we expect a bunch of ENOENT because the directory does not exist.
-	isExpectedErr := func(err error) bool {
-		return err != nil && strings.HasSuffix(err.Error(), "no such file or directory")
-	}
-	if !isExpectedErr(result.ASNDatabaseErr) {
-		t.Fatal("unexpected error", result.ASNDatabaseErr)
-	}
-	if !isExpectedErr(result.CABundleErr) {
-		t.Fatal("unexpected error", result.CABundleErr)
-	}
-	if !isExpectedErr(result.CountryDatabaseErr) {
-		t.Fatal("unexpected error", result.CountryDatabaseErr)
-	}
-	if !isExpectedErr(result.RmdirErr) {
-		t.Fatal("unexpected error", result.RmdirErr)
-	}
-}
-
-func TestCleanupWithEmptyInput(t *testing.T) {
-	result, err := Cleanup("")
-	if !errors.Is(err, ErrEmptyDir) {
-		t.Fatal("unexpected error", err)
-	}
-	if result != nil {
-		t.Fatal("expected nil result")
-	}
-}