update_engine: remove unused variable am: 17be0f93a8 am: 04051cd445

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

Change-Id: I7d9f34d294dd5e1bb1cfa1664e9ca25800f7ba81
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>