update_engine: remove unusued offset_ am: 25a3a30e22 am: c1bbabcdd3

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

Change-Id: Icf05a513ced86a2b2fe0098b74b6811235b870d3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>