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.Graphics.Metal/BackgroundResources.cs

@@ -1,107 +0,0 @@
-using SharpMetal.Metal;
-using System;
-using System.Collections.Generic;
-using System.Runtime.Versioning;
-using System.Threading;
-
-namespace Ryujinx.Graphics.Metal
-{
-    [SupportedOSPlatform("macos")]
-    class BackgroundResource : IDisposable
-    {
-        private readonly MetalRenderer _renderer;
-
-        private CommandBufferPool _pool;
-        private PersistentFlushBuffer _flushBuffer;
-
-        public BackgroundResource(MetalRenderer renderer)
-        {
-            _renderer = renderer;
-        }
-
-        public CommandBufferPool GetPool()
-        {
-            if (_pool == null)
-            {
-                MTLCommandQueue queue = _renderer.BackgroundQueue;
-                _pool = new CommandBufferPool(queue, true);
-                _pool.Initialize(null); // TODO: Proper encoder factory for background render/compute
-            }
-
-            return _pool;
-        }
-
-        public PersistentFlushBuffer GetFlushBuffer()
-        {
-            _flushBuffer ??= new PersistentFlushBuffer(_renderer);
-
-            return _flushBuffer;
-        }
-
-        public void Dispose()
-        {
-            _pool?.Dispose();
-            _flushBuffer?.Dispose();
-        }
-    }
-
-    [SupportedOSPlatform("macos")]
-    class BackgroundResources : IDisposable
-    {
-        private readonly MetalRenderer _renderer;
-
-        private readonly Dictionary<Thread, BackgroundResource> _resources;
-
-        public BackgroundResources(MetalRenderer renderer)
-        {
-            _renderer = renderer;
-
-            _resources = new Dictionary<Thread, BackgroundResource>();
-        }
-
-        private void Cleanup()
-        {
-            lock (_resources)
-            {
-                foreach (KeyValuePair<Thread, BackgroundResource> tuple in _resources)
-                {
-                    if (!tuple.Key.IsAlive)
-                    {
-                        tuple.Value.Dispose();
-                        _resources.Remove(tuple.Key);
-                    }
-                }
-            }
-        }
-
-        public BackgroundResource Get()
-        {
-            Thread thread = Thread.CurrentThread;
-
-            lock (_resources)
-            {
-                if (!_resources.TryGetValue(thread, out BackgroundResource resource))
-                {
-                    Cleanup();
-
-                    resource = new BackgroundResource(_renderer);
-
-                    _resources[thread] = resource;
-                }
-
-                return resource;
-            }
-        }
-
-        public void Dispose()
-        {
-            lock (_resources)
-            {
-                foreach (BackgroundResource resource in _resources.Values)
-                {
-                    resource.Dispose();
-                }
-            }
-        }
-    }
-}