Remove included file string_util.h from payload_signer.cc
This is to remove the direct dependency on libchrome.
Remove unused header base/string/string_util.h.
Bug: 360917504
Test: build
Change-Id: I78447f9e7130235d7af2e9aa783e543181474172
diff --git a/payload_generator/payload_signer.cc b/payload_generator/payload_signer.cc
index 4b24eba..b661a92 100644
--- a/payload_generator/payload_signer.cc
+++ b/payload_generator/payload_signer.cc
@@ -23,7 +23,6 @@
#include <base/logging.h>
#include <base/strings/string_number_conversions.h>
-#include <base/strings/string_util.h>
#include <brillo/data_encoding.h>
#include <openssl/err.h>
#include <openssl/pem.h>