objects: remove vlc_player_t as a valid public c++ obj cast

Since the player became an opaque type, the object cast should not be
available publicly anymore.

include/vlc_objects.h

@@ -89,7 +89,6 @@ static inline struct vlc_object_t *VLC_OBJECT(struct t *d) \
 
 vlc_object_cast(libvlc_int_t)
 vlc_object_cast(intf_thread_t)
-vlc_object_cast(vlc_player_t)
 vlc_object_cast(stream_t)
 vlc_object_cast(stream_directory_t)
 vlc_object_cast(stream_extractor_t)

src/player/player.h

@@ -280,6 +280,8 @@ struct vlc_player_t
     struct vlc_player_timer timer;
 };
 
+vlc_object_cast(vlc_player_t);
+
 #ifndef NDEBUG
 /*
  * Assert that the player mutex is locked.