mb/google/octopus: remove unused variant_board_id()

The variant_board_id() API was never used on octopus because all
boards in the octopus family used the EC to get board id, i.e. they
all use EC_GOOGLE_CHROMEEC_BOARDID. Therefore, remove the code
and declarations so as not to cause confusion.

BUG=b:111808427,b:111743717
TEST=built

Change-Id: I4f9a24b46dd4262120075d3d42daf22015a3dd50
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/27635
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
3 files changed