Porting away from kinit/klauncher

- Remove check for kded and confupdate stages in SplashApp, those setStage()
  calls were removed a long time ago[1]

- Rename 'kinit' stage to 'startPlasma', since it's called from startplasma

- Remove startKDEInit(), it was declared but not defined anywhere, most
  likely left-over from a previous refactor.

https://phabricator.kde.org/R297:185afa8a3af8002172d8f5d6aa08c57244422a12