Scrub some old migration code.
Review Request #3672 - Created April 9, 2016 and submitted
|nhoward_tw, patricklaw, zundel|
These migrations have been over for a very long time. Leaves the config migration script intact, as we may still need it from time to time, but removes its huge list of old migrations that everyone already applied long ago. Also a couple of drive-by style fixes while I happened to be poking at some files.
CI passed: https://travis-ci.org/pantsbuild/pants/builds/121938493