firmware/2lib: Remove dead kernel verification function references

Remove references to non-existent kernel verification functions from
public headers and comments. These functions (vb2api_get_kernel_size,
vb2api_kernel_load_vblock, etc.) were either renamed or removed in
previous refactors but their declarations in comments and return codes
remained.

Updated comments to correctly reference vb2api_load_kernel().

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

Change-Id: I8f00ad7dabac66688072585aa729d82370ac2f12
Signed-off-by: Jakub Czapiga <czapiga@google.com>
Cq-Depend: chromium:7637740
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7632946
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Commit-Queue: Jeffery Miller <jefferymiller@google.com>