Remove files that are now part of the GraphViz package

[schnellecom]

Because Digraphs is now an extension of the GraphViz package.

Connected to gap-packages/GraphvizForGAP#43.

[schnellecom]

Results from discussion in the GAPDays 2026 Fall:

The goal for the structure of the packages is as follows:

• GraphvizForGAP provides functions, which are used to visualise graphs
• Digraphs has code, which uses these functions so that a user can generate these visualisations
• Digraphs has GraphvizForGAP as an extension, so in particular only loads the code using the GraphvizForGAP functions if the GraphvizForGAP is loaded. As a result Digraphs can be used without installing GraphvizForGAP
• The main reason for this is that the packages dealing with the mathematical objects maintain their own visualisation themselves, to avoid that GraphvizForGAP contains many things from many packages and becomes very hard to maintain

As a result this PR is now no longer applicable to how we want to structure the packages.
Did I miss anything @james-d-mitchell @fingolfin ?