Remove unused field save_rollback_data am: 399bd4dae1

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3391041

Change-Id: Ib86d0adf2cb418b95b9954617e551a7773501dcf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>