Fix namespace descriptions (#129)

dotnet/xml/ns-Microsoft.Extensions.ML.xml

@@ -1,6 +1,5 @@
 <Namespace Name="Microsoft.Extensions.ML">
   <Docs>
     <summary>Utilities for extending the ML.NET API, including the prediction engine pool</summary>
-    <remarks>To be added.</remarks>
   </Docs>
 </Namespace>

dotnet/xml/ns-Microsoft.ML.Vision.xml

@@ -1,6 +1,5 @@
 <Namespace Name="Microsoft.ML.Vision">
   <Docs>
     <summary>Namespace containing image classification components.</summary>
-    <remarks>To be added.</remarks>
   </Docs>
 </Namespace>