stored_playlist: renamed and moved spl_delete() to stored_playlist.c

The function deletePlaylist() shouldn't be in playlist.c.

src/stored_playlist.h

@@ -53,6 +53,9 @@ spl_move_index(const char *utf8path, unsigned src, unsigned dest);
 enum playlist_result
 spl_clear(const char *utf8path);
 
+enum playlist_result
+spl_delete(const char *name_utf8);
+
 enum playlist_result
 spl_remove_index(const char *utf8path, unsigned pos);