[usdMtlx] Removing some unnecessary comments in UsdMtlxReader and rem…

…oving use of deprecated GetConnectedSource() from FileFormat test

Fixes #1636

(Internal change: 2243217)

pxr/usd/usdMtlx/reader.cpp

@@ -1495,21 +1495,6 @@ _Context::AddShaderNode(const mx::ConstNodePtr& mtlxShaderNode)
         return UsdShadeShader();
     }
 
-    // XXX -- If the node def is implemented by a nodegraph we may need
-    //        to reference that node graph instead of creating a
-    //        UsdShadeShader.  That will require other USD support for
-    //        inline shaders.
-
-    // XXX -- At the moment it's not clear how we'll handle a nodedef
-    //        backed by a nodegraph.  Will it be opaque with the
-    //        implementation known only to the shader registry?  Or will
-    //        using that shader cause the nodegraph to be added to the
-    //        stage?  In that case do we make a separate copy of the
-    //        nodegraph for each use or reference a single instantation?
-    //
-    //        Note that we don't have a efficient way to get the nodegraph
-    //        that implements the nodedef here.
-
     // Choose the name of the shader.  In MaterialX this is just
     // mtlxShaderNode->getName() and has no meaning other than to uniquely
     // identify the shader.  In USD to support materialinherit we must

pxr/usd/usdMtlx/testenv/testUsdMtlxFileFormat.py

@@ -142,7 +142,8 @@ def test_MultiBindInputs(self):
         for inputName, source in inputToSource.items():
             input = nodeGraph.GetInput(inputName)
             self.assertEqual(input.HasConnectedSource(), True)
-            self.assertEqual(input.GetConnectedSource()[0].GetPath(), source)
+            self.assertEqual(
+                input.GetConnectedSources()[0][0].source.GetPath(), source)
 
     def test_MultiOutputNodes(self):
         """
@@ -159,9 +160,9 @@ def test_MultiOutputNodes(self):
 
         for path, connNodeName, connectionName in testInfo:
             node = UsdShade.Shader.Get(stage, path)
-            conn = node.GetInput('in').GetConnectedSource()
-            self.assertEqual(conn[0].GetPrim().GetPath().name, connNodeName)
-            self.assertEqual(conn[1], connectionName)
+            conn = node.GetInput('in').GetConnectedSources()[0][0]
+            self.assertEqual(conn.source.GetPath().name, connNodeName)
+            self.assertEqual(conn.sourceName, connectionName)
 
     def test_nodesWithoutNodegraphs(self):
         """