[kwin] Drop testapp for detecting whether direct rendering works on glx

The main purpose of the opengl testapp was to set the environment
variable LIBGL_ALWAYS_INDIRECT if direct rendering is not supported
before glx gets initialized.

With Qt5 we may no longer set this environment variable. QtQuick
requires direct rendering. On IvyBridge QtQuick is crashing if the
variable is set. Thus we are no longer allowed to set it and thus the
complete test becomes pointless.

The test app basically whitelisted most drivers anyway, the only
drivers which were problematic are the proprietary Catalyst drivers.
It that's still a problem we can also disable OpenGL compositing on
those drivers through the recommendation in the GLPlatform.

This also means that the KWIN_DIRECT_GL variable is no longer useful.