Clean up egl/glx extensions in debug console

Summary:
As there is no difference between egl and glx extensions anymore inside
KWin (both are provided through the OpenGLBackend) there is no need to
have to group boxes and do a manual hide/show based on which platform is
used.

Instead there is now just one group box and it's called "Platform
Extensions".

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3400