[xabt] remove leftover/unshipped MSBuild files (#10344)

Fixes: #10343

Removes MSBuild targets/props files that are not shipped in installers and not used in .NET 6+ projects.

## Files removed:

- `src/Xamarin.Android.Build.Tasks/Xamarin.Android.PCLSupport.targets`
- `src/Xamarin.Android.Build.Tasks/Xamarin.Android.PCLSupport.props`
- `src/Xamarin.Android.Build.Tasks/Xamarin.Android.FSharp.targets`
- `src/Xamarin.Android.Build.Tasks/Xamarin.Android.VisualBasic.targets`

Co-authored-by: Jonathan Peppers <[email protected]>

Author: Copilot

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

@@ -112,21 +112,9 @@
     <None Include="Xamarin.Android.EmbeddedResource.targets">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
-    <None Include="Xamarin.Android.FSharp.targets">
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-    </None>
-    <None Include="Xamarin.Android.PCLSupport.props">
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-    </None>
-    <None Include="Xamarin.Android.PCLSupport.targets">
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-    </None>
     <None Include="Xamarin.Android.Tooling.targets">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
-    <None Include="Xamarin.Android.VisualBasic.targets">
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-    </None>
   </ItemGroup>
   <ItemGroup>
     <None Include="ManifestOverlays\Timing.xml">