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/UI/Views/Settings/SettingsGraphicsView.axaml

@@ -8,7 +8,6 @@
     xmlns:ui="clr-namespace:FluentAvalonia.UI.Controls;assembly=FluentAvalonia"
     xmlns:ext="clr-namespace:Ryujinx.Ava.Common.Markup"
     xmlns:viewModels="clr-namespace:Ryujinx.Ava.UI.ViewModels"
-    xmlns:helper="clr-namespace:Ryujinx.Common.Helper;assembly=Ryujinx.Common"
     Design.Width="1000"
     mc:Ignorable="d"
     x:DataType="viewModels:SettingsViewModel">
@@ -34,24 +33,16 @@
                                 ToolTip.Tip="{ext:Locale SettingsTabGraphicsBackendTooltip}"
                                 Text="{ext:Locale SettingsTabGraphicsBackend}"
                                 Width="250" />
-                        <ComboBox
-                            Name="GraphicsBackendSelector"
-                            Width="350"
-                            HorizontalContentAlignment="Left"
-                            ToolTip.Tip="{ext:Locale SettingsTabGraphicsBackendTooltip}"
-                            SelectedIndex="{Binding GraphicsBackendIndex}">
-                            <ComboBoxItem IsVisible="{Binding IsVulkanAvailable}" ToolTip.Tip="{ext:Locale SettingsTabGraphicsBackendAutoTooltip}" >
-                                <TextBlock Text="{ext:Locale SettingsTabGraphicsBackendAuto}" />
-                            </ComboBoxItem>
+                        <ComboBox Width="350"
+                                HorizontalContentAlignment="Left"
+                                ToolTip.Tip="{ext:Locale SettingsTabGraphicsBackendTooltip}"
+                                SelectedIndex="{Binding GraphicsBackendIndex}">
                             <ComboBoxItem IsVisible="{Binding IsVulkanAvailable}">
                                 <TextBlock Text="Vulkan" />
                             </ComboBoxItem>
                             <ComboBoxItem IsEnabled="{Binding IsOpenGLAvailable}">
                                 <TextBlock Text="OpenGL" />
                             </ComboBoxItem>
-                            <ComboBoxItem IsEnabled="{x:Static helper:RunningPlatform.IsArmMac}">
-                                <TextBlock Text="Metal (ARM Mac only, Experimental)" />
-                            </ComboBoxItem>
                         </ComboBox>
                     </StackPanel>
                     <StackPanel Orientation="Horizontal" IsVisible="{Binding IsVulkanSelected}">