launcher-service: Remove obsolete --directory='' from hint

Inheriting the current working directory for the game's s-r-l-s has been the default when injecting debugging commands into game containers since mid 2022.

When communicating with s-r-l-s --alongside-steam or Flatpak, the service's current working directory is uninteresting, and trying to preserve the client's current working directory is usually a better fit anyway.

So we can reduce visual noise here.