spthui: kill obsolete gdk_threads_enter() vs. spthui_lock() swap comment

We don't do any UI manipulation in do_add_playlists() after

  cc101ec spthui: do not clear the playlist list at do_add_playlists()

and that's been a while.

We've since gotten rid of all the drop-spthui-lock-grab-gdk-lock
games and nowadays just offload UI work to gdk_threads_add_idle()
if we're holding spthui_lock() and thus cannot do it directly.