Revert "vcl::Window::SupportsDoubleBuffering: respect VCL_DOUBLEBUFFERING_FORCE_ENABLE"

This reverts commit 7df3879d. It's not
necessary since commit f16d82ea (make it
possible to force enable double buffering for all widgets, 2015-06-03).

vcl/source/window/window.cxx

@@ -3936,8 +3936,7 @@ vcl::RenderSettings& Window::GetRenderSettings()
 
 bool Window::SupportsDoubleBuffering() const
 {
-    static bool bDoubleBuffering = getenv("VCL_DOUBLEBUFFERING_FORCE_ENABLE");
-    return mpWindowImpl->mbDoubleBuffering || bDoubleBuffering;
+    return mpWindowImpl->mbDoubleBuffering;
 }
 
 void Window::SetDoubleBuffering(bool bDoubleBuffering)