Move DB driver specification out of config and into code

[melange396]

In our API server, the database driver/adapter for SQLAlchemy is specified as part of a URI. Specifically, it is in the "scheme" segment of the URI (what i typically think of as the "protocol", the part before the ://). Currently we use mysql+mysqldb which corresponds to the mysqlclient dialect.

In the server code, it is read from the environment here. It is specified in CI and in the development Makefile, but in prod, this comes from the "vault" in delphi-ansible-web. We should not leave this up to a configuration parameter which can be hidden inside an encrypted and somewhat inaccessible location, but instead do it one place in the code for consistency and transparency.