[engine_step] Remove vestigial _asdict method.
[engine_step] Remove vestigial _asdict method.

Things switched to attr.asdict, but this was left behind.

R=yiwzhang

Bug: 1127089
Change-Id: I9f1ff92bb9764ca865f32b9ac209d6c2dbd7184d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/2435754
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/recipes-py,luci/recipes-py