host: Remove dead SignatureBuf

SignatureBuf() in host/lib/signature_digest.c has no callers in the
tree. Remove it along with its declaration in signature_digest.h and
unused OpenSSL/RSA headers, and clean up a copy-pasted error prefix in
pad_digest_utility.c.

BUG=b:124141368
TEST=make runtests
BRANCH=none
TAG=agy
CONV=2efa1fd4-161e-459c-95ac-49f549088989

Change-Id: I231a76ea692bcf13013f2ef57389c4b132823d72
Signed-off-by: Jakub Czapiga <czapiga@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/8497138
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Commit-Queue: chromeos-auto-retry@chromeos-bot.iam.gserviceaccount.com <chromeos-auto-retry@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>