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/StringHelper.cs

@@ -1,30 +0,0 @@
-using SharpMetal.Foundation;
-using SharpMetal.ObjectiveCCore;
-using System.Runtime.Versioning;
-
-namespace Ryujinx.Graphics.Metal
-{
-    [SupportedOSPlatform("macos")]
-    class StringHelper
-    {
-        public static NSString NSString(string source)
-        {
-            return new(ObjectiveC.IntPtr_objc_msgSend(new ObjectiveCClass("NSString"), "stringWithUTF8String:", source));
-        }
-
-        public static unsafe string String(NSString source)
-        {
-            char[] sourceBuffer = new char[source.Length];
-            fixed (char* pSourceBuffer = sourceBuffer)
-            {
-                ObjectiveC.bool_objc_msgSend(source,
-                    "getCString:maxLength:encoding:",
-                    pSourceBuffer,
-                    source.MaximumLengthOfBytes(NSStringEncoding.UTF16) + 1,
-                    (ulong)NSStringEncoding.UTF16);
-            }
-
-            return new string(sourceBuffer);
-        }
-    }
-}