image_signing: delete old crosh_workarounds logic
image_signing: delete old crosh_workarounds logic

We deleted crosh workarounds logic years ago (e.g. CL:178932),
so drop references here too.

BUG=None
TEST=precq passes
BRANCH=None

Change-Id: I2b195a7ca863101be35a4bfde29d7950a96c1edd
Reviewed-on: https://chromium-review.googlesource.com/907069
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: David Riley <a1c52a99f156ff41f0ea478f29291144551a02f6@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot