Don't clean up temp directory

Summary:
This seriously speeds up cmf startup.
I can't actually find where those temp files are being written - either they're not in /tmp or they're deleted somewhere else. Anyway, half the startup time of cmf is caused by deleting these temp files.
It might be better if we could have a more efficient way to read from a string, of course, but this is an easy win for now.

Test Plan: ran cmf --profile

Reviewers: pad

Reviewed By: pad

CC: whhone

Differential Revision: https://phabricator.fb.com/D749201

Task ID: 2178569