Revert "remove unused function"

This reverts commit a19205e3ad.

src/apps.js

@@ -305,6 +305,14 @@ function validateRobotsTxt(robotsTxt) {
     return null;
 }
 
+function validateBackupFormat(format) {
+    assert.strictEqual(typeof format, 'string');
+
+    if (format === 'tgz' || format == 'rsync') return null;
+
+    return new AppsError(AppsError.BAD_FIELD, 'Invalid backup format');
+}
+
 function validateLabel(label) {
     if (label === null) return null;