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

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

Change-Id: Ib86b3c107bbc9eea4bdd169e9d7b0eb265aeab85
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>