I wanted to get rid of legacy course files too. I wrote a script to move them off to Amazon S3 and then delete them from the course, and delete them from the disk if they were not used by any courses any more. It's not 100% what you are after (i.e. you aren't looking to move them elsewhere), but have a look at the code in that script;
https://moodle.org/mod/forum/discuss.php?d=251233
Martin