kservice: remove InitialPreference
Since this was a KDE specific feature it caused problems with interoperability. Applications based on other XDG implementations would entirely ignore it and by extension have a different idea of what application should be used to open a given mimetype. Instead we now rely only on the XDG provided facilities to manage preference. Specifically plasma-desktop is meant to ship a kde-mimeapps.list that encodes the Plasma default preferences and distributions are meant to ship *mimeapps.list to encode sensible preferences beyond that.