vboot: remove VbExEcEnteringMode and friends

After informing EC of the mode chosen by vboot, it is stored
in a global and never accessed again.  Remove this function,
its calls, and its tests.

Also note some significant issues which existed:
(1) Using accessor VbGetMode for tests, rather than just
    overriding VbExEcEnteringMode when needed.
(2) The tests checking the value sent to VbExEcEnteringMode
    (vboot_api_kernel2_tests and vboot_detach_menu_tests)
    actually call the function themselves.

BUG=b:124141368, chromium:1014379
TEST=make clean && make runtests

Change-Id: Ib8e510a1e1c663bb3f8238a9ad15e3e64d7350b0
Signed-off-by: Joel Kitching <kitching@google.com>
Cq-Depend: chromium:1864533
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1830239
Reviewed-by: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Julius Werner <jwerner@chromium.org>
6 files changed