This file hasn't been updated in over 6 years.  The backwards compat
logic in here dates back to pre-R20 releases, and the only boards we
released that long ago are EOL now.  So scrub all the unused code to
simplify it.

While we're here, fix a few style issues, and update the docs.

Bug: 997354
Test: `cros help` still works
Change-Id: Id226cd73f045972b88e7bc5926a415f567d1e41c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1875488Reviewed-by: Mike Nichols <mikenichols@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>