vboot/vboot_kernel: move workbuf init into vb2_load_partition

Not needed prior to that.

This CL is part of a series to merge vboot1 and vboot2.0
kernel verification code; see b/181739551.

BUG=b:181739551
TEST=make clean && make runtests
BRANCH=none

Signed-off-by: Joel Kitching <kitching@google.com>
Change-Id: Ia60aa05384384bc1e2981266c33b960115734fae
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2882527
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>