pt empty db class

What does this MR do?

cleans out some stuff from Db.php

How confident are you it won't break things if deployed?

hmmmm... quite actually, but there have been many lines touched....

Checklist

• added a test, or explain why one is not needed/possible... (only refactor, nothing new)
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))