fix usage of default-filemanager in lumina-open

[Nanolx]

Found one minor mistake in your transistion from desktop-settings to session-settings.

Now if default-filemanager is unset or invalid lumina-fm is used, else default-filemanager is used.

[beanpole135]

Thanks for finding that!
I just missed removing a couple of the QSettings lines from before.
Now, if the extension is "directory" it will go into the cmdFromUser() function, which reads the default application for the extension and uses it. In the case of a directory, it will default to running "lumina-fm" if there is no default file manager set.