Revert the Metal Experiment (#701)

Metal sounded like a good idea to get in the emulator but frankly I
underestimated just how experimental and not ready it was.
From my write up in the Discord:
```
As is, Metal supports only a few games.
The games it does support freeze on first use of not playing them via Vulkan, because shader translation is broken.
So you need to use a dirty hack to not delete all your shaders.
Not to mention it breaks many games via MoltenVK because of changes to the shared GPU code.

Merging Metal seemed like a great idea, because of the few games it does support.
But I don't think it's worth it. Many of the games it breaks via MoltenVK *don't work via Metal*. 
Which effectively makes current Ryubing worse for Mac users than Ryujinx 1.1.1403.

I think what I'm gonna do is revert Metal, and reopen it as a PR. That way, you can still take advantage of the Metal backend as is, but without making other games worse with no solution.
```

For what it's worth, the shader translation part could at least be
"fixed" by always applying a 30ms delay for shader translation to Metal.
That being said, that solution sucks ass.
The MoltenVK regressions are even worse.



I hope this is not a let down to the Mac users. I hope you realize I'm
reverting this because you're actively getting a worse experience with
it in the emulator.

src/Ryujinx/Utilities/Configuration/ConfigurationState.cs

@@ -318,15 +318,14 @@ namespace Ryujinx.Ava.Utilities.Configuration
 
         private static GraphicsBackend DefaultGraphicsBackend()
         {
-            // Any system running macOS should default to auto, so it uses Vulkan everywhere and Metal in games where it works well. 
-            if (OperatingSystem.IsMacOS())
-                return GraphicsBackend.Auto;
-
-            // Any system returning any amount of valid Vulkan devices should default to Vulkan.
+            // Any system running macOS or returning any amount of valid Vulkan devices should default to Vulkan.
             // Checks for if the Vulkan version and featureset is compatible should be performed within VulkanRenderer.
-            return VulkanRenderer.GetPhysicalDevices().Length > 0 
-                ? GraphicsBackend.Vulkan 
-                : GraphicsBackend.OpenGl;
+            if (OperatingSystem.IsMacOS() || VulkanRenderer.GetPhysicalDevices().Length > 0)
+            {
+                return GraphicsBackend.Vulkan;
+            }
+
+            return GraphicsBackend.OpenGl;
+        }
+            }
         }
-    }
-}