[generator] Fix <remove-attr> metadata.

[jpobst]

Fixes #976.

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, resulting in:

<package name='my.package'>
  <class />
</package>

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.

Fix <remove-attr> to work as expected.