nongnu: steam: Reduce dependencies.

• nongnu/packages/steam-client.scm (steam-client-libs)
  (steam-gameruntime-libs, steam): Reduce dependencies to a minimal set.
  (make-internal-script): Remove pulseaudio symlink.

This merge request reduces the steam package dependencies down to the smallest set possible to still allow steam and its games to run, and adds comments to the remaining dependencies for why they are needed.