[Xamarin.Android.Build.Tasks] stop using proguard-android.txt from …

…Android SDK tools

Context: https://discord.com/channels/732297728826277939/732297837953679412/1303383272918618163

A customer found their app grew in size when building on Windows
versus macOS.

The difference being a file that was passed to the `<R8/>` task on
Windows, but not on macOS:

    C:\Users\[username]\AppData\Local\Android\Sdk\tools\proguard\proguard-android.txt

`tools` is *ancient*, as any Android SDK should be using
`cmdline-tools` now.

Let's stop using this file as a default set of proguard rules.

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1899,7 +1899,6 @@ because xbuild doesn't support framework reference assemblies.
     <_ProguardConfiguration Include="$(ProguardConfigFiles)" />
   </ItemGroup>
   <ItemGroup Condition=" '$(ProguardConfigFiles)' == '' ">
-    <_ProguardConfiguration Include="$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt"     Condition=" Exists ('$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt') " />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_xamarin.cfg"         Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(_ProguardProjectConfiguration)"                               Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_project_primary.cfg" Condition=" '$(AndroidLinkTool)' != '' " />