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 for them"; this would be a Consulting/Support issue and not come upstream today. I can't identify any arguments for retaining this workflow there, at least.

Test Plan:

• Grepped for files purge, got nothing.
• Grepped for purge, looked for anything that looked like instructions or documentation, got nothing.

I don't recall recommending anyone run this script in many years, and didn't even remember that it existed or what it did when T12948 was reported, so I believe it is not in widespread use.

Reviewers: joshuaspence, chad

Reviewed By: joshuaspence

Maniphest Tasks: T12948

Differential Revision: https://secure.phabricator.com/D18384