Start KDE Cleanups

Summary:
Remove outdated parts of startkde that are no longer used

Remove setion about kwin_failsafe which isn't used
Remove code related to oxygen fonts that aren't used
Remove section about darwin
Get rid of check that dbus-server exists, it autostarts
Don't try to set $qdbus to a path to qdbus... it wasn't used
conherently, so we don't really need it

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: mart, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2894