a/d3d: Delete non-existent keyed_mutex param from doc comment in d3d12_d3d_allocator.hpp

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2577>

src/xrt/auxiliary/d3d/d3d_d3d12_allocator.hpp

···
  * @param xsci Swapchain create info: note that the format is assumed to be a DXGI_FORMAT (conversion to typeless is
  * automatic)
  * @param image_count The number of images to create.
- * @param keyed_mutex Whether to create images with a shared "keyed mutex" as well
  * @param[out] out_images A vector that will be cleared and populated with the images.
  * @param[out] out_handles A vector that will be cleared and populated with the corresponding native handles.
  * @param[out] out_image_mem_size The image memory allocation size in bytes