TF-1608 Add references and inReplyTo fields to email offline

(cherry picked from commit 1f58a814a5003c3cbd1f3ba287de5bb54565d52f)

model/lib/email/email_property.dart

@@ -20,6 +20,5 @@ class EmailProperty {
   static const String headers = 'headers';
   static const String headerMdnKey = 'Disposition-Notification-To';
   static const String messageId = 'messageId';
-  static const String inReplyTo = 'inReplyTo';
   static const String references = 'references';
 }