Cleanup color scheme manager part of KCM Deco code

Summary:
PreviewCient is internal to the KCM, it's not used by actual decorations
where we need API compatibility.
This property is not used by the KCM.

The current code parses every color scheme config file for every
decoration. This is one of the big contributing factors to this KCM
being a bit sluggish.

It was effectively deprecated in 2015 when a decoration palette was
introduced. If we wanted to add colorscheme functionality in the KCM,
we'd use that instead at a previewclient level.

Test Plan:
Found this was a problem with hotspot
Grepped code for usage
Ran KCM

Reviewers: Plasma

Subscribers: plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9239