vboot/ui: Remove VB2_SCREEN_BLANK

Remove the unused VB2_SCREEN_BLANK from vb2_screen enum.

BUG=none
TEST=emerge-asurada depthcharge
TEST=make runtests
BRANCH=none

Cq-Depend: chromium:2793450
Change-Id: I931f6ad804f6da3b710bef9d800c26960d20c3cb
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2793448
Reviewed-by: Julius Werner <jwerner@chromium.org>