Remove unnecessary last_error_ field am: 8ab0f41066 am: ce7af8b115

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

Change-Id: I68204447b5f546cf5198e86a6c1b3b79a63171c4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>