prefs: return success if nothing to delete am: 6a00fe9b1d am: 99e1f2b85a

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3035052

Change-Id: I935aa764e78ec1356ac7baa36ed92622a61473d6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>