firmware/2lib: Remove unused vb2_sha256_extend function

Remove vb2_sha256_extend() implementation and declaration as it is no
longer used in the project (only by its own unit tests).

BRANCH=main
BUG=b:124141368
TEST=make run2tests

Change-Id: I6025cc381dc2af8f48fe2643f32f3593fee6c473
Signed-off-by: Jakub Czapiga <czapiga@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7632947
Reviewed-by: Grzegorz Bernacki <bernacki@google.com>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>