Removed redundant ContentTypeDescriptor update in Save()

The `Save()` method currently has two places where it updates the `_ContentTypeDescriptors` cache. This is redundant, and means the second one—which is actually more cautious—will never actually fire. To avoid this, I've removed the first one.

While I was at it, I also updated the second one to use pattern matching syntax for the variable definition, which is a bit less code, and easier to read.

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -320,13 +320,6 @@ public virtual int Save([ValidatedNotNull, NotNull]Topic topic, bool isRecursive
         );
       }
 
-      /*------------------------------------------------------------------------------------------------------------------------
-      | Update content types collection, if appropriate
-      \-----------------------------------------------------------------------------------------------------------------------*/
-      if (topic is ContentTypeDescriptor && !contentTypeDescriptors.Contains(topic.Key)) {
-        contentTypeDescriptors.Add((ContentTypeDescriptor)topic);
-      }
-
       /*------------------------------------------------------------------------------------------------------------------------
       | Ensure derived topic is set
       >-------------------------------------------------------------------------------------------------------------------------
@@ -364,11 +357,11 @@ public virtual int Save([ValidatedNotNull, NotNull]Topic topic, bool isRecursive
       \-----------------------------------------------------------------------------------------------------------------------*/
       if (
         topic.IsNew &&
-        topic is ContentTypeDescriptor &&
+        topic is ContentTypeDescriptor descriptor &&
         _contentTypeDescriptors != null &&
         !_contentTypeDescriptors.Contains(topic.Key)
       ) {
-        _contentTypeDescriptors.Add((ContentTypeDescriptor)topic);
+        _contentTypeDescriptors.Add(descriptor);
       }
 
       /*------------------------------------------------------------------------------------------------------------------------