Remove dune xt data

This MR removes the dependency on dune-xt-data. dune-xt only uses the Spe10 part of dune-xt-data and now downloads the relevant files during cmake configuration instead.

This will break dune-gdt, so we might want to wait until I removed the kinetic stuff from gdt. On the other hand, gdt is already broken with master, so it doesn't really matter if we break it even more :).