Remove managed files with 0 uses:

1. Find all of them with...

   select file_managed.uri from file_managed left join file_usage on file_managed.fid = file_usage.fid where file_usage.fid is null;

2. Export a csv. 

3. Use regex to escape spaces...

   s/ /\\ /

4. Use regex to mv files to a backup directory (e.g. "orphans")...

   s/".*:\/\/\(.*\)"/mv web\/sites\/default\/files\/\1 ..\/orphans

5. Delete them from the file_managed table with...

   DELETE file_managed from file_managed left join file_usage on file_managed.fid = file_usage.fid where file_usage.fid is null;