D23504: [platforms/x11] Remove triple buffering detection

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REVISION SUMMARY
  It is not clear what the advantage of triple buffering is for KWin. An X11
  compositor is meant to swap buffers once every frame. For that triple
  buffering is not necessary.
  
  The functionality is not maintained, does not reliably work as displayed by
  the existence of an environment variable to force some behavior, pollutes
  our code and every compositing-related problem that might be mitigated with
  triple buffering should find a simpler and more fitting solution with other
  means.
  
  There is one caveat which is if we shall block for retrace. We set it
  currently according to the result of the swap profiler and in the most common
  case with double buffering it is set to true. But on Nvidia systems this might
  be actual the wrong behavior. Instead of trying to work around this ignore
  the issue for now and move the overall architecture to something less complex
  by presenting after paint how we do it in the Wayland DRM backend and with
  double buffering on GLX (although this is at the moment also borken because
  we actually present then twice).

TEST PLAN
  kwin_x11 tested on i915.

REPOSITORY
  R108 KWin

BRANCH
  rmTripleBuffering

REVISION DETAIL
  https://phabricator.kde.org/D23504

AFFECTED FILES
  platformsupport/scenes/opengl/CMakeLists.txt
  platformsupport/scenes/opengl/backend.h
  platformsupport/scenes/opengl/swap_profiler.cpp
  platformsupport/scenes/opengl/swap_profiler.h
  plugins/platforms/x11/common/eglonxbackend.cpp
  plugins/platforms/x11/common/eglonxbackend.h
  plugins/platforms/x11/standalone/glxbackend.cpp
  plugins/platforms/x11/standalone/glxbackend.h

To: romangg, #kwin
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