db/update/Walk: clear Song::in_playlist

Without clearing all `in_playlist` flags, the songs will never be
revealed again if they were hidden once by a CUE sheet, not even after
the CUE sheet gets deleted or modified.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1784

src/db/update/Walk.cxx

@@ -531,6 +531,7 @@ UpdateWalk::Walk(Directory &root, const char *path, bool discard) noexcept
 
 	{
 		const ScopeDatabaseLock protect;
+		root.ClearInPlaylist();
 		PurgeDanglingFromPlaylists(root);
 	}