cros-ec-board: clear out useless entries

This list is only used to check the EC boards to build for on
pre-unibuild boards.  Unibuild boards use cros_config_host instead.

Nuke all the unibuild boards which got added to this list, and add a
little message informing others the list does not need extended.

BUG=none
TEST=CQ dry run

Change-Id: I9b54df719b23d08bb596266ae1f6e805d537def5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2284186
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
1 file changed