ChangeNotesState: Remove redundant revertOf field

This is a @Column in Change and thus belongs in ChangeColumns. In
ChangeNotes#create it reuses the same value in both places, so they
should always be equal, modulo missing ChangeColumns, in which case the
field in ChangeNotesState should never be accessed.

Change-Id: I63eb5acdcd6c47ea2ceea7d830d5b5b6e4b28d2f

java/com/google/gerrit/server/notedb/ChangeNotes.java

@@ -679,7 +679,7 @@ public class ChangeNotes extends AbstractChangeNotes<ChangeNotes> {
   }
 
   public Change.Id getRevertOf() {
-    return state.revertOf();
+    return state.columns().revertOf();
   }
 
   public boolean hasReviewStarted() {