Restructured submodules

- Now the repository 'jutge-dockerfiles' is the one having a Git submodule for its 3 dependencies (`jutge-vinga`, `jutge-toolkit` and `jutge-server-toolkit`).
- Two of them were submodules here, and therefore have been deleted.
- Now only two submodules remain: `driver-src` and `dockerfiles`, which I've moved to the root.