Remove unnecessary last_error_ field am: 8ab0f41066

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3391042

Change-Id: I11270f53ede80da674e6a93aac01306dc3a4529b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>