[ksmserver] Restore legacy sessions

Summary:
KSMServer has two modes, the slightly ancient mode (XSMP), and the
really ancient legacy mode (XSM).

When startup was rewritten in 96a595f1b8602838f065f312bd5b6ae24f8d6ac1
we accidentally dropped the call that invokes restore of the legacy
session as well as the normal mode.

BUG: 404318

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19028