Remove old code for metrics calculations

[pdmnyberg]

Describe the bug
There is a middleware that was used for supplying the metrics data. It uses static files instead of the database. Keeping it in the project can make reading the code confusing.

When all data is represented in the database, consider removing: https://github.com/elixir-europe-training/Training-Metrics-Database/blob/main/dash_app/middleware.py