drop unused models (build, criterion, settings, user), code & assets
The 4 mentioned DB models haven't been used in years. Drop them to make the
codebase and schema easier to maintain. The related functions (from controllers
etc) are dropped as well.
This also removes:
* `EnumSymbol` and related classes, because it's not used anywhere and we
already use SQLAlchemy's provided Enum which seems to work well
* `util/koji_interface.py`, no longer required
* some outdated images
Note that there are still some unused util functions lying around, like
`check_api_rights()` or `check_api_key()`, which I decided to keep in case they
could still be useful in the future.