Remove included file string_split.h from payload_signer.cc am: 4e1be09cc6

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3304585

Change-Id: I34b9094309061c16eada980bbc67aad3dedbea9d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>