don't reset dc->command in quitDecode()

The decoder thread is responsible for resetting dc->command after a
command was executed.  As a consequence, we can assume that
dc->command is already NONE after decoder_stop().

src/player_thread.c

@@ -35,7 +35,6 @@ static void quitDecode(void)
 {
 	dc_stop(&pc.notify);
 	pc.state = PLAYER_STATE_STOP;
-	dc.command = DECODE_COMMAND_NONE;
 	pc.command = PLAYER_COMMAND_NONE;
 	wakeup_main_task();
 }