graphics: libobs-d3d11: Use DXGI_SWAP_EFFECT_FLIP_DISCARD on Windows 10

This is supposed to eliminate a copy by DWM with extra benefits for
borderless fullsceen, which should help the fullscreen projector.

libobs/graphics/graphics.c

@@ -1272,9 +1272,6 @@ gs_swapchain_t *gs_swapchain_create(const struct gs_init_data *data)
 	if (!gs_valid_p("gs_swapchain_create", data))
 		return NULL;
 
-	if (new_data.num_backbuffers == 0)
-		new_data.num_backbuffers = 1;
-
 	return graphics->exports.device_swapchain_create(graphics->device,
 							 &new_data);
 }