(maint) Remove unused .testseries file

The .testseries file was added to puppet 4 years ago and has not been
touched since, and there's no explanation as to why it's there. To
reduce the amount of unused cruft that's being packed around in the
source, this commit removes that file.
Merge pull request #1995 from adrienthebo/maint/remove-testseries-file

(maint) Remove unused .testseries file