Delete 'demo_api' module #579

The demo_api module was initially written so that a demo version of the Murfey server could be run without making RabbitMQ calls. However, it is very cumbersome to keep up to date with all the API changes that are being made.

There are already elements in place within the server startup code that we could make use of so that the message sending object is left inactive when Murfey is started up in demo mode, and this would allow us to completely remove the demo_api module.