chroot_version_hooks.d: avoid using parallel_emerge

The wrapper is going away, and in most cases using it over emerge
isn't performance critical.  Switch to plain emerge to simplify.

BUG=chromium:998918
TEST=None

Change-Id: I15d5d3dc905e68441a0971a299a66420efeedbc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1773848
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
6 files changed