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

Currently rebase_path() function takes up to three parameters, so
passing a fourth won't work.

This was probably due to the work done in:
https://chromium.googlesource.com/chromium/src/+/ee55eaceea8d7632a59761c603af0f4503db00df

BUG=434539
TEST=gn help rebase_path
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/892043004

Cr-Commit-Position: refs/heads/master@{#314097}

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"