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/PersistentFlushBuffer.cs

@@ -1,100 +0,0 @@
-using Ryujinx.Graphics.GAL;
-using SharpMetal.Metal;
-using System;
-using System.Runtime.Versioning;
-
-namespace Ryujinx.Graphics.Metal
-{
-    [SupportedOSPlatform("macos")]
-    internal class PersistentFlushBuffer : IDisposable
-    {
-        private readonly MetalRenderer _renderer;
-
-        private BufferHolder _flushStorage;
-
-        public PersistentFlushBuffer(MetalRenderer renderer)
-        {
-            _renderer = renderer;
-        }
-
-        private BufferHolder ResizeIfNeeded(int size)
-        {
-            BufferHolder flushStorage = _flushStorage;
-
-            if (flushStorage == null || size > _flushStorage.Size)
-            {
-                flushStorage?.Dispose();
-
-                flushStorage = _renderer.BufferManager.Create(size);
-                _flushStorage = flushStorage;
-            }
-
-            return flushStorage;
-        }
-
-        public Span<byte> GetBufferData(CommandBufferPool cbp, BufferHolder buffer, int offset, int size)
-        {
-            BufferHolder flushStorage = ResizeIfNeeded(size);
-            Auto<DisposableBuffer> srcBuffer;
-
-            using (CommandBufferScoped cbs = cbp.Rent())
-            {
-                srcBuffer = buffer.GetBuffer();
-                Auto<DisposableBuffer> dstBuffer = flushStorage.GetBuffer();
-
-                if (srcBuffer.TryIncrementReferenceCount())
-                {
-                    BufferHolder.Copy(cbs, srcBuffer, dstBuffer, offset, 0, size, registerSrcUsage: false);
-                }
-                else
-                {
-                    // Source buffer is no longer alive, don't copy anything to flush storage.
-                    srcBuffer = null;
-                }
-            }
-
-            flushStorage.WaitForFences();
-            srcBuffer?.DecrementReferenceCount();
-            return flushStorage.GetDataStorage(0, size);
-        }
-
-        public Span<byte> GetTextureData(CommandBufferPool cbp, Texture view, int size)
-        {
-            TextureCreateInfo info = view.Info;
-
-            BufferHolder flushStorage = ResizeIfNeeded(size);
-
-            using (CommandBufferScoped cbs = cbp.Rent())
-            {
-                MTLBuffer buffer = flushStorage.GetBuffer().Get(cbs).Value;
-                MTLTexture image = view.GetHandle();
-
-                view.CopyFromOrToBuffer(cbs, buffer, image, size, true, 0, 0, info.GetLayers(), info.Levels, singleSlice: false);
-            }
-
-            flushStorage.WaitForFences();
-            return flushStorage.GetDataStorage(0, size);
-        }
-
-        public Span<byte> GetTextureData(CommandBufferPool cbp, Texture view, int size, int layer, int level)
-        {
-            BufferHolder flushStorage = ResizeIfNeeded(size);
-
-            using (CommandBufferScoped cbs = cbp.Rent())
-            {
-                MTLBuffer buffer = flushStorage.GetBuffer().Get(cbs).Value;
-                MTLTexture image = view.GetHandle();
-
-                view.CopyFromOrToBuffer(cbs, buffer, image, size, true, layer, level, 1, 1, singleSlice: true);
-            }
-
-            flushStorage.WaitForFences();
-            return flushStorage.GetDataStorage(0, size);
-        }
-
-        public void Dispose()
-        {
-            _flushStorage.Dispose();
-        }
-    }
-}