gn: Do not advertise a rebase_path() command with more than 3 parameters.

tools/gn/function_rebase_path.cc

Index: tools/gn/function_rebase_path.cc
diff --git a/tools/gn/function_rebase_path.cc b/tools/gn/function_rebase_path.cc
index 0dde36af81c1f83707460d595708946afae47f4a..cad850f7e073ae7bb57aa1d2c98f544b5275f5a6 100644
--- a/tools/gn/function_rebase_path.cc
+++ b/tools/gn/function_rebase_path.cc
@@ -185,10 +185,6 @@ const char kRebasePath_Help[] =
     "  # Might produce \"D:\\source\\project\\myfile.txt\" on Windows or\n"
     "  # \"/home/you/source/project/myfile.txt\" on Linux.\n"
     "\n"
-    "  # Convert a file's path separators from forward slashes to system\n"
-    "  # slashes.\n"
-    "  foo = rebase_path(\"source/myfile.txt\", \".\", \".\", \"to_system\")\n"
-    "\n"
     "  # Typical usage for converting to the build directory for a script.\n"
     "  action(\"myscript\") {\n"
     "    # Don't convert sources, GN will automatically convert these to be\n"