Cleanup old code

[pdelboca]

This is a long-overdue cleanup of the codebase. The code removed is no longer required and it is messing around with the documentation tasks.

What is being removed?

The extension shouldn't use the database for IATIFiles as we are not tracking information nor we need to persist anything. The flow of the extension should be simple:

1. Create a Dataset with a namespace.
2. Add the resources with IATIFileTypes.
3. Generate the IATI File .
4. Create the new resource if sucess, otherwise throw a report with the errors.

We do not store invalid activity files, we are not validating any independent IATIFileType and we do not track attempts to generate activity/organisation files.

Therefore, there is no need to have a database table for this extension.

[germankay]

@pdelboca I tested the entire IATI file loading and generation process locally and it worked correctly