Majolica: remove CONFIG_DIAGNOSTIC_UI

Remove CONFIG_DIAGNOSTIC_UI in defconfig and make it default enabled

BUG=b:190796342
BRANCH=none
TEST=build locally

Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Change-Id: I1525f0c07a139e0ccd7e347a77d907436aac8a8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/depthcharge/+/3023806


Reviewed-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-by: Karthikeyan Ramasubramanian <kramasub@chromium.org>