lars: Remove SDHC boot device entry

LARs design don't have SD Connector over SD 3.0 Ctrl rather
using USB interface, hence need to disable SDHC bootable
entry from list.

BUG=chrome-os-partner:48190
BRANCH=None
TEST=Build & boot LARs. SD over USB booting fine

Change-Id: I420d4471163a574c656c29ccede0b39531c31850
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/315381
Reviewed-by: Aaron Durbin <adurbin@chromium.org>