[build] remove unused Xamarin.Android files (#9719)

* [build] remove unused Xamarin.Android files

Two files cause some confusion by *existing* in the repo:

* `Xamarin.Android.Sdk.props`
* `Xamarin.Android.Sdk.targets`

Neither are shipped in `C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\`,
for example. They are effectively dead code.

If we remove them, this means more stuff can be removed:

* `@(XATargetsSrcFiles)` item group

* `<SetVsMonoAndroidRegistryKey/>` MSBuild task

* `create-vsix` and `create-pkg` projects

* `Xamarin.Android.Bindings.Before.targets` and `Xamarin.Android.Bindings.After.targets`

* `Xamarin.Android.Common.Before.targets` and `Xamarin.Android.Common.After.targets`

Removing these files may improve MSBuild project evaluation time, as
they are currently running several `Exists()` checks that always
return `false`.

In the Xamarin.Android days, the XamarinVS Visual Studio extension
layed down custom MSBuild targets files *inside* the Xamarin.Android
installation to augment the build. This doesn't happen any more in
.NET 6+, as they do not lay down files inside a .NET workload in
`C:\Program Files\dotnet\packs`. It wouldn't make sense to do so.

Note that I moved `runtime-entitlements.plist` to
`build-tools/installers/` as it is still used.

Author: jonathanpeppers