[startkde] Rename method startKSMServer to reflect real behaviour
Summary:
Since ksmserver was split, start-plasma launches plasma-session which in
turn invokes ksmserver.
Test Plan: No code changes, only a method rename
Reviewers: Plasma, broulik, apol
Reviewed By: Plasma, broulik, apol
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27621