backups: remove limit object from storage config

this is causing UI to get confused

src/backups.js

@@ -238,7 +238,7 @@ async function setState(id, state) {
 }
 
 async function startBackupTask(auditSource) {
-    let error = locker.lock(locker.OP_FULL_BACKUP);
+    const error = locker.lock(locker.OP_FULL_BACKUP);
     if (error) throw new BoxError(BoxError.BAD_STATE, `Cannot backup now: ${error.message}`);
 
     const backupConfig = await getConfig();