[kcmkwin/compositing] Do not write GLPlatformInterface config

Summary:
The gui element to select the platform interface is dropped. Thus we
should not store the config for it. This is rather important as changing
compositor settings on Wayland would result in egl being written into the
config which potentially breaks an X11 session.

BUG: 378114

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5285