Remove AUTHORS

AUTHORS isn't really relevant in the days of git.  devstack has no
formal release where it would make sense to auto-generate an AUTHORS
file either.

If you really want to see who contributes, you can run

 $ git log --format='%aN <%aE>' | sort -f | uniq > AUTHORS

Change-Id: I9e154f7fdf814b4bd21a9d4cf82e46ec3db2f1ca
Merge "Remove AUTHORS"