Revert "launch autostart apps in ksmserver using KRun"

KRun::runApplication will show blocking error dialogs if it fails to
find the executable
This means we don't autostart the next app, which could be fatal if it
comes before...

Summary:
...kwin/plasma
We shouldn't be having blocking calls in ksmserver it can deadlock
And even in the best case we'd still end up blocking ksplash for 30
seconds

We then port to KProcess which was part of the motivation behind the
patch as it
has better stdout handling

This reverts commit 0f19e92f.

BUG: 379254

Also added port of autostarting applications to KProcess
It has better stdout handling

Test Plan: Logged in, still got my main session

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5618