don't join the tags table here, it makes the query slower

Hi guys, as discussed in the IRC, we were joining the tags table [here](https://github.com/diaspora/diaspora/blob/develop/app/models/status_message.rb#L176), which is unnecessary.

This should improve the performance of the /followed_tags page. I've taken some measurements on my local machine with [this](https://gist.github.com/4649079) script (takes quite some time to run) which is a variation of the seeds.

I have taken like 20 samples and in average the response time of the json is improved by 1 sec, I would change this anyway being the join unnecessary.

Cheers!