Remove included file string_util.h from payload_signer.cc am: a9a2513870 am: 2e893ad8c4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3309435

Change-Id: I642b269bffbe72ccf85881e172b31772a8d612f6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>