update_engine: remove is_rollback am: ad1eaead8f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3203012

Change-Id: I73df5090eda187e7c31fc529d462b96254298c31
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>