Metadata `<remove-attr>` doesn't work as expected

[jpobst]

Given an api.xml element like:

<package name='my.package'>
  <class name='MyClass' api-since='29' />
</package>

You would expect to be able to remove the api-since attribute with this metadata:

<remove-attr path='/api/package[@name='my.package']/class[@name='MyClass']' name='api-since' />

However, even though generator reads the name attribute, it then proceeds to ignore it and remove all attributes on the matched node. This seems unintuitive, and I'm not sure how this could ever be successfully used in a binding project, since leaving an empty <class>, etc. node causes generator to crash.

Relevant code:
https://github.com/xamarin/java.interop/blob/0c90cf5cd67c5bd4978fbcf016281d330426989f/src/Java.Interop.Tools.Generator/Metadata/FixupXmlDocument.cs#L149-L167

<remove-attr> seems to be undocumented: https://docs.microsoft.com/en-us/xamarin/android/platform/binding-java-library/customizing-bindings/java-bindings-metadata