Don’t document SpacePen viets as valid gutter decoration items

We should really be retrieving the view for the item from the view
registry, but that can be done in a different PR.

src/custom-gutter-component.coffee

@@ -98,7 +98,6 @@ class CustomGutterComponent
       delete @decorationItemsById[decorationId]
 
       if newItem
-        # `item` should be either an HTMLElement or a space-pen View.
         newItemNode = null
         if newItem instanceof HTMLElement
           newItemNode = newItem

src/text-editor-presenter.coffee

@@ -547,7 +547,7 @@ class TextEditorPresenter
   #     decoration.id : {
   #       top: # of pixels from top
   #       height: # of pixels height of this decoration
-  #       item (optional): HTMLElement or space-pen View
+  #       item (optional): HTMLElement
   #       class (optional): {String} class
   #     }
   #   }