HelloXR Vulkan version enables USE_MIRROR_WINDOW default, and it is annoying when testing it on Windows

[utzcoz]

See https://github.com/KhronosGroup/OpenXR-SDK-Source/blob/main/src/tests/hello_xr/graphicsplugin_vulkan.cpp#L21.

From the comment, USE_MIRROR_WINDOW shows a dummy window when running Vulkan version of HelloXR on Windows platform, and this window is used for RenderDoc. As a normal tester, I don't use RenderDoc, and I just want the Runtime to display a native window to show the rendered content. What about disabling it default? If other folks want to run it with RenderDoc, they can build it with patch to enable USE_MIRROR_WINDOW macro.