Drop klauncher usage from KCrash

Summary:
As per other discussions, kdeinit has limited gains and is not widely
deployed in KDE application. KInit as a framework is on its way out.

As such using klauncher to restart a crashed application has no benefit,
we can drop it and simplify the code significantly.

It also means restarted apps stay within any namespaces set up by the
launcher.

Test Plan:
killall -SEGV on plasmashell and dolphin
Both restarted as expected
The direct code path already existed, so should be safe to use

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28336