Remove unused AnnotationController#isComment method

The last thing that used this was removed in 7829cc1.

h/static/scripts/directive/annotation.coffee

@@ -83,14 +83,6 @@ AnnotationController = [
     this.tagsAutoComplete = (query) ->
       $q.when(tags.filter(query))
 
-    ###*
-    # @ngdoc method
-    # @name annotation.AnnotationController#isComment.
-    # @returns {boolean} True if the annotation is a comment.
-    ###
-    this.isComment = ->
-      not (model.target?.length or model.references?.length)
-
     ###*
     # @ngdoc method
     # @name annotation.AnnotationController#isHighlight.