[generator] Fix <remove-attr/> metadata (#999)

Fixes: #976
Given an `api.xml` element like:
<package name='my.package'>
<class name='MyClass' api-since='29' />
</package>
One would expect to be able to remove the `class/@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.
Update `<remove-attr/>` to work as expected, removing the named
attribute.

Author: jpobst

src/Java.Interop.Tools.Generator/Metadata/FixupXmlDocument.cs

@@ -153,7 +153,7 @@ public void Apply (ApiXmlDocument apiDocument, string apiLevelString, int produc
 varmatched=false;
 
 foreach(varnodeinnodes){
-node.RemoveAttributes();
+node.Attributes(name).Remove();
 matched=true;
 }
 

tests/generator-Tests/Unit-Tests/FixupXmlDocumentTests.cs

@@ -82,7 +82,19 @@ public void RemoveAttribute ()
 
 api.ApplyFixupFile(fixup);
 
-Assert.AreEqual("<api><package /><package name='java' jni-name='java' /></api>",api.ApiDocument.ToString(SaveOptions.DisableFormatting).Replace('\"','\''));
+Assert.AreEqual("<api><package jni-name='android' /><package name='java' jni-name='java' /></api>",api.ApiDocument.ToString(SaveOptions.DisableFormatting).Replace('\"','\''));
+}
+
+[Test]
+publicvoidRemoveNotFoundAttribute()
+{
+// Attribute 'foo' doesn't exist on node
+varapi=GetXmlApiDocument();
+varfixup=GetFixupXmlDocument("<remove-attr path=\"/api/package[@name='android']\" name='foo' />");
+
+api.ApplyFixupFile(fixup);
+
+Assert.AreEqual("<api><package name='android' jni-name='android' /><package name='java' jni-name='java' /></api>",api.ApiDocument.ToString(SaveOptions.DisableFormatting).Replace('\"','\''));
 }
 
 [Test]