Remove google/urara mainboard

Remove google/urara mainboard

This board never really existed and nobody has any hardware left over
for it.

Change-Id: Icdba4f5209725995e4a55dcdbc299a9e91a5869a
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36490
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>