[Visualization] Remove default Metal device option

Remove the default Metal device option, as it can return nil on systems
where it won't be expected to work.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Visualization/SpectrumView.m

@@ -47,7 +47,6 @@ extern NSString *CogPlaybackDidStopNotficiation;
 - (id)initWithFrame:(NSRect)frame {
 	NSDictionary *sceneOptions = @{
 		SCNPreferredRenderingAPIKey: @(SCNRenderingAPIMetal),
-		SCNPreferredDeviceKey: MTLCreateSystemDefaultDevice(),
 		SCNPreferLowPowerDeviceKey: @(NO)
 	};