Cleanup ActivatePeer

[jonpryor]

Commit cba6137 added JniValueManager.ActivatePeer():

public abstract void ActivatePeer (
        IJavaPeerable? self,
        JniObjectReference reference,
        ConstructorInfo cinfo,
        object? []? argumentValues);

However, no code paths actually provide self; when self isn't null, that's a different code path:

https://github.com/xamarin/java.interop/blob/main/src/Java.Interop/Java.Interop/ManagedPeer.cs#L107-L110

The self parameter should be removed.