Remove save all playlists action

It's hard-coded to m3u, needs a new select directory dialog with option to select different playlist formats.

Fixes #987

src/playlist/playlistmanager.h

@@ -223,8 +223,6 @@ class PlaylistManager : public PlaylistManagerInterface {
   // Rate current song using 0 - 5 scale.
   void RateCurrentSong2(const int rating) override;
 
-  void SaveAllPlaylists();
-
  private slots:
   void SetActivePlaying() override;
   void SetActivePaused() override;