Remove obsolete files

This MR aims to remove all files that are not needed anymore (fixes #37 (closed)). I already removed distribute_testing.py and .travis.ctest. Other candidates are:

• python scripts:
  • dxt_time_traces.py
  • generate_compare_functions.py
  • numa_speedup.py
• files in the root directory
  • .travis.cfg.in
  • .codecov.yml
  • stamp-vc (and if we remove this, also .gitattributes becomes obsolete)
  • .mailmap
  • dune-xt.pc.in
  • doc folder (I don't think we have a working Doxygen documentation?)

@r_milk01, @falbr_01, could you have a look at this list and see which of these files are still needed?