daemon: quit Plymouth without transition if only XDMCP is allowed

When GDM is configured as a standalone XDMCP server, the manager quits Plymouth by running plymouth quit --retain-splash. This is not ideal because there is no transition to a local X server. The terminal can be then left from the Plymouth run in the graphics mode with no getty prompt and also disallowing switching to another VT. The patch fixes the problem by instead running plymouth quit which always switches the terminal to the text mode.