Move #to_discussion to NoteOnDiff

app/models/concerns/note_on_diff.rb

@@ -28,4 +28,8 @@ module NoteOnDiff
   def can_be_award_emoji?
     false
   end
+
+  def to_discussion
+    Discussion.new([self])
+  end
 end

app/models/diff_note.rb

@@ -107,10 +107,6 @@ class DiffNote < Note
     self.noteable.find_diff_discussion(self.discussion_id)
   end
 
-  def to_discussion
-    Discussion.new([self])
-  end
-
   private
 
   def supported?

app/models/legacy_diff_note.rb

@@ -53,10 +53,6 @@ class LegacyDiffNote < Note
     self.line_code
   end
 
-  def to_discussion
-    Discussion.new([self])
-  end
-
   # Check if this note is part of an "active" discussion
   #
   # This will always return true for anything except MergeRequest noteables,