update_engine: remove unusued offset_ am: 25a3a30e22

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3248445

Change-Id: Id39b15ac9a32e5f5ea913164fd8da4ec0a543591
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>