R1:8443366f32d3

Remove `bin/files purge` workflow

Summary:
Fixes T12948. See that task for substantial discussion and context. Briefly:

- This workflow is very old, and won't work for large (>2GB) files.
- This workflow has become more dangerous than it once was, and can fail in several ways that delete data and/or make recovery much more difficult (see T12948 for more discussion).
- This was originally added in D6068, which is a bit muddled, but looks like "one install ran into a weird issue so I wrote a script…