Remove included file string_split.h from payload_signer.cc am: 4e1be09cc6 am: 4fafd39545

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

Change-Id: Ic214f7674d2d0588cef73c54c645958c4b409535
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>