merge dune-xt into dune-gdt

As discussed offline with @r_milk01, we consider to merge dune-xt into dune-gdt.

Pros:

• less projects, less repos, less super repos
• (CI) dependency simplification (no need to commit dune-xt in gdt-super, wait for base images update, commit dune-gdt, ...)
• you clone one repo and get everything

Cons:

• wheels would get too big, need to be split into several ones
• no standalone dune-xt without dune-gdt:
  • add a bindings_dune_xt only target if someone wants only those bindings?

Please voice your opinions, @all!