Remove database steps in DEVELOPER-ADVANCED.md

I just went through the developer install guide and noticed that the postgres and redis sample files aren't being used anymore. Read through [this](https://meta.discourse.org/t/enough-with-the-sample-files/10351) thread on meta as well so I'm assuming discourse is moving away from using sample files?

I also removed "add vagrant user" since it was in the vagrant section further down the doc. FWIW, I didn't do steps 7 & 8 and was able to get it running great.