Remove unused helper scripts from bin/tests/system/

The following discussion from !985 (merged) should be addressed:

• @michal started a discussion: (+1 comment)

  I may be confused, but I do not really see why we still need the following scripts:

  • bin/tests/system/start.sh
  • bin/tests/system/stop.sh
  • bin/tests/system/stopall.sh
  • bin/tests/system/setup.sh

  I would take a shot at removing them in a separate MR. AFAICT, only stop.sh is actively used (in the rrsetorder test), but it should be easy to use stop() from conf.sh.common instead.