Remove not needed file attribute

It was typically the same as the "name" one, just with a .rng suffix,
i.e. redundant.

Change-Id: I8abb296b2ee963e214971ff748fd5b079320dfa9

writerfilter/documentation/ooxml/model.rng

@@ -256,9 +256,6 @@ documentation for that file. The schema has two parts:
   <define name="namespace-element">
     <element name="namespace">
       <attribute name="name"/>
-      <optional>
-        <attribute name="file"/>
-      </optional>
       <zeroOrMore>
         <ref name="start-element"/>
       </zeroOrMore>

writerfilter/source/ooxml/factoryimpl_ns.py

@@ -714,8 +714,7 @@ using namespace com::sun::star;
 
     files = {}
     for nsNode in getChildrenByName(modelNode, "namespace"):
-        if nsNode.hasAttribute("file"):
-            files[nsNode.getAttribute("file")] = nsNode
+        files[nsNode.getAttribute("name")] = nsNode
 
     for nsNode in [i for i in getChildrenByName(modelNode, "namespace") if i.getAttribute("name") == nsName]:
         nsLabel = nsToLabel(nsNode)