Carefully delete unused apps

Some unused apps remain in this repo, leftovers from Freedom Coop's fork (the upstream for this repo).

The ones I know should be deleted are faircoin, general, and multi-currency. They cause test failures, which you can reproduce by doing
```
./manage.py test
```
"Carefully" means that before and after deleting the app, you need to look for leftover references in the apps that remain.