update_engine: remove unused variable am: 17be0f93a8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2924565

Change-Id: I313d521b63bb6e17528422741ddfd095a75262c8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>