Remove neglected/redundant AUTHORS file #2328

This file was last updated 9 years ago when only the 2 project founders
were contributors. We have now had around 30 contributors but we still
have only 2 listed in this file. As we have also had contributor
guidelines indicating for interested parties to contribute to this file
for some time there is clear indication that it is redundant.
Especially as we primarily use git / GitHub for attribution.