[opengl] Drop functionality to start kcmshell compositing

Summary:
The idea of opening the compositing kcm was to show a warning. But that
broke quite some time ago without noticing. We had two ways:

• pass through --args command line argument
• use dbus call to already open kcm

Neither of the two ways is working. The kwincompositing doesn't parse
the arguments and the dbus interface doesn't exist any more.

Following the advice to remove functionality nobody noticed that it is
broken, this is removed with this change. This probably broke with
introducing the new KCM which happened IIRC for Plasma 5.0.

BUG: 393845
FIXED-IN: 5.14.0

Reviewers: KWin, Plasma

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13235