MBS-6059: Remove obsolete features

[PhMemmel]

Closes #47
Closes #55

This is still a draft, there are some places in the code still using the xml we need to work around. They are currently marked with "TODO" comments

[PhMemmel]

I first tried to remove the XML completely, but realized it is being used to extract the information we need (result variables, feedback etc.) from the PHP side. I could not find a way to extract that from the base64 we are storing, because we do not have a PHP routine converting the base64 into a GGB xml. This can only be done by the JS GGB API. So for now I kept the XML in the frontend (JS, HTML) which is then being submitted to the PHP scripts which then extract all the necessary information, do the validations, calculate the mark etc.

However, I removed the XML storage as well as all the "GGB exercise" related code. The applet is only being injected via the base64 string from the database. As soon as this is loaded, the JS module extracts the XML via GGB JS API call into an HTML hidden input field which - when submitted - can be used to extract all the information from.

As a first approach I think this is fine. However, we could think about extracting all the necessary information in the JS module and create our own data structure there which then is being submitted to the PHP scripts. This would make the whole PHP structure much more easier and readable and we could make much more use of the GGB JS API itself for extracting the information we need. Any thoughts on this @mputzi and anyone else?

After some manual testing I think I did not screw up too much of the functionality by removing these things, so this would be a good step to continue the refactoring development.