Remove support for GraphML witnesses?

[michael-schwarz]

In all categories where we produce witnesses, SV-COMP will remove support for GraphML-witnesses this year. We should also disable them in our SV-COMP setting already now as constructing them is costly (c.f. #1322) and issues with these may hide other unsoundnesses or issues in the nightly rides.

While we need ARG for abstract debugging etc, I personally also don't see any reason for keeping support for producing GraphML files. Maybe we should remove them altogether?

[michael-schwarz]

I have now disabled the support in the SV-COMP config (#1733). This leaves the question of whether we want to completely remove support.