Phaser: Remove support for early board versions

At this point, support for the early board versions can be removed.
This leaves the board code cleaner, and provides a better starting
point for any follower projects.

BUG=b:128413812
BRANCH=octopus
TEST=builds

Change-Id: I0eda3ecc5b33bfe0721a0fa9d59f7884a0d3732e
Signed-off-by: Diana Z <dzigterman@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1525066
Reviewed-by: Karthikeyan Ramasubramanian <kramasub@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>