API v2: Improve status index performance

Always return all statuses, don't filter by statuses used in visible projects.

According to @apfelfabrik it should be ok to just return all statuses instead of only the visible ones.

![screen shot 2013-11-15 at 16 55 37](https://f.cloud.github.com/assets/188986/1551296/957c88bc-4e11-11e3-8dba-99b067b6955f.png)

The change reduces the duration for a single `/api/v2/statuses.json` request on a test instance from about 6 seconds to 0.13 seconds.